diff --git a/web/src/App.svelte b/web/src/App.svelte
index ffaee33..2e086dc 100644
--- a/web/src/App.svelte
+++ b/web/src/App.svelte
@@ -9,11 +9,13 @@
     type ShantenResponse,
     ALL_TILES,
   } from './lib/agari';
+  import { t } from './lib/i18n';
   import TilePalette from './lib/components/TilePalette.svelte';
   import ContextOptions from './lib/components/ContextOptions.svelte';
   import ScoreResult from './lib/components/ScoreResult.svelte';
   import Tile from './lib/components/Tile.svelte';
   import DoraPicker from './lib/components/DoraPicker.svelte';
+  import LanguageSwitcher from './lib/components/LanguageSwitcher.svelte';
 
   // ============================================================================
   // State
@@ -623,19 +625,20 @@
         <span class="logo-icon">🀄</span>
         <span class="logo-text">Agari</span>
       </h1>
-      <p class="tagline">Riichi Mahjong Calculator</p>
+      <p class="tagline">{$t.tagline}</p>
+      <LanguageSwitcher />
     </div>
   </header>
 
   <main class="main">
     {#if wasmError}
       <div class="error-banner">
-        <span>⚠️ Failed to load calculator: {wasmError}</span>
+        <span>⚠️ {$t.failedToLoad} {wasmError}</span>
       </div>
     {:else if !wasmLoaded}
       <div class="loading-banner">
         <div class="spinner"></div>
-        <span>Loading calculator...</span>
+        <span>{$t.loadingCalculator}</span>
       </div>
     {:else}
       <div class="calculator-layout">
@@ -643,9 +646,9 @@
         <div class="hand-section">
           <div class="card">
             <div class="card-header">
-              <h2>Build Your Hand</h2>
+              <h2>{$t.buildYourHand}</h2>
               <button class="btn btn-secondary" onclick={clearHand}>
-                Clear
+                {$t.clear}
               </button>
             </div>
 
@@ -659,25 +662,25 @@
 
             <!-- Meld Builder Buttons -->
             <div class="meld-buttons">
-              <span class="meld-label">Add Meld:</span>
-              <button class="btn btn-sm" onclick={() => startMeldBuilder('chi')} disabled={showMeldBuilder}>Chi</button>
-              <button class="btn btn-sm" onclick={() => startMeldBuilder('pon')} disabled={showMeldBuilder}>Pon</button>
-              <button class="btn btn-sm" onclick={() => startMeldBuilder('kan')} disabled={showMeldBuilder}>Open Kan</button>
-              <button class="btn btn-sm" onclick={() => startMeldBuilder('ankan')} disabled={showMeldBuilder}>Closed Kan</button>
+              <span class="meld-label">{$t.addMeld}</span>
+              <button class="btn btn-sm" onclick={() => startMeldBuilder('chi')} disabled={showMeldBuilder}>{$t.chi}</button>
+              <button class="btn btn-sm" onclick={() => startMeldBuilder('pon')} disabled={showMeldBuilder}>{$t.pon}</button>
+              <button class="btn btn-sm" onclick={() => startMeldBuilder('kan')} disabled={showMeldBuilder}>{$t.openKan}</button>
+              <button class="btn btn-sm" onclick={() => startMeldBuilder('ankan')} disabled={showMeldBuilder}>{$t.closedKan}</button>
             </div>
 
             <!-- Meld Builder Panel -->
             {#if showMeldBuilder}
               <div class="meld-builder">
                 <div class="meld-builder-header">
-                  <span>Building {meldBuilderType === 'ankan' ? 'Closed Kan' : meldBuilderType === 'kan' ? 'Open Kan' : meldBuilderType.charAt(0).toUpperCase() + meldBuilderType.slice(1)}</span>
+                  <span>{#if meldBuilderType === 'chi'}{$t.buildingChi}{:else if meldBuilderType === 'pon'}{$t.buildingPon}{:else if meldBuilderType === 'kan'}{$t.buildingOpenKan}{:else}{$t.buildingClosedKan}{/if}</span>
                   <span class="meld-builder-hint">
                     {#if meldBuilderType === 'chi'}
-                      (select 3 sequential tiles of the same suit)
+                      {$t.hintChi}
                     {:else if meldBuilderType === 'pon'}
-                      (select 3 of the same tile)
+                      {$t.hintPon}
                     {:else}
-                      (select 4 of the same tile)
+                      {$t.hintKan}
                     {/if}
                   </span>
                 </div>
@@ -695,13 +698,13 @@
                   {/each}
                 </div>
                 <div class="meld-builder-actions">
-                  <button class="btn btn-sm btn-secondary" onclick={cancelMeldBuilder}>Cancel</button>
+                  <button class="btn btn-sm btn-secondary" onclick={cancelMeldBuilder}>{$t.cancel}</button>
                   <button
                     class="btn btn-sm btn-primary"
                     onclick={confirmMeld}
                     disabled={meldBuilderTiles.length !== (meldBuilderType === 'kan' || meldBuilderType === 'ankan' ? 4 : 3)}
                   >
-                    Add Meld
+                    {$t.confirmAddMeld}
                   </button>
                 </div>
               </div>
@@ -712,7 +715,7 @@
           {#if melds.length > 0}
             <div class="card">
               <div class="melds-display">
-                <h3 class="melds-title">Called Melds</h3>
+                <h3 class="melds-title">{$t.calledMelds}</h3>
                 <div class="melds-list">
                   {#each melds as meld, index (meld.id)}
                     <div class="meld-group">
@@ -735,9 +738,9 @@
           <!-- Hand Display -->
           <div class="card">
             <div class="hand-header">
-              <h3>Your Hand</h3>
+              <h3>{$t.yourHand}</h3>
               {#if handTiles.length > 0}
-                <span class="winning-tile-hint">Click a tile to select it as winning tile</span>
+                <span class="winning-tile-hint">{$t.selectWinningTileHint}</span>
               {/if}
             </div>
             <div class="hand-tiles-selectable">
@@ -751,7 +754,7 @@
                   >
                     <Tile tile={entry.tile} red={entry.isRed} size="md" />
                     {#if selectedWinningTileIndex === index}
-                      <div class="winning-badge">WIN</div>
+                      <div class="winning-badge">{$t.winBadge}</div>
                     {/if}
                   </button>
                   <button
@@ -775,11 +778,11 @@
               {#if shantenResult.success}
                 <div class="shanten-display">
                   {#if shantenResult.shanten === -1}
-                    <span class="shanten-badge complete">✓ Complete</span>
+                    <span class="shanten-badge complete">✓ {$t.complete}</span>
                   {:else if shantenResult.shanten === 0}
-                    <span class="shanten-badge tenpai">Tenpai</span>
+                    <span class="shanten-badge tenpai">{$t.tenpai}</span>
                   {:else}
-                    <span class="shanten-badge">{shantenResult.shanten}-shanten</span>
+                    <span class="shanten-badge">{shantenResult.shanten}{$t.shanten}</span>
                   {/if}
                   <span class="shanten-type">({shantenResult.best_type})</span>
                 </div>
@@ -797,14 +800,14 @@
               class="dora-toggle"
               onclick={() => showDoraSection = !showDoraSection}
             >
-              <span>Dora Indicators</span>
+              <span>{$t.doraIndicators}</span>
               <span class="toggle-arrow" class:open={showDoraSection}>▼</span>
             </button>
 
             {#if showDoraSection}
               <div class="dora-content">
                 <div class="dora-row">
-                  <label class="dora-label">Dora</label>
+                  <span class="dora-label">{$t.dora}</span>
                   <div class="dora-tiles">
                     {#each doraIndicators as entry, index (entry.id)}
                       <div class="dora-tile-wrapper">
@@ -826,14 +829,14 @@
                         type="button"
                         class="dora-add-btn"
                         onclick={() => showDoraPicker = true}
-                      >+ Add</button>
+                      >{$t.addButton}</button>
                     {/if}
                   </div>
                 </div>
 
                 {#if isRiichi}
                   <div class="dora-row">
-                    <label class="dora-label">Ura Dora</label>
+                    <span class="dora-label">{$t.uraDora}</span>
                     <div class="dora-tiles">
                       {#each uraDoraIndicators as entry, index (entry.id)}
                         <div class="dora-tile-wrapper">
@@ -855,7 +858,7 @@
                           type="button"
                           class="dora-add-btn"
                           onclick={() => showUraDoraPicker = true}
-                        >+ Add</button>
+                        >{$t.addButton}</button>
                       {/if}
                     </div>
                   </div>
@@ -863,7 +866,7 @@
 
                 {#if akaCount > 0}
                   <div class="aka-display">
-                    Aka Dora in hand: <span class="aka-count">{akaCount}</span>
+                    {$t.akadoraInHand} <span class="aka-count">{akaCount}</span>
                   </div>
                 {/if}
               </div>
@@ -890,7 +893,7 @@
 
           <!-- Results (moved from right column) -->
           <div class="card results-card">
-            <h2 class="card-title">Results</h2>
+            <h2 class="card-title">{$t.results}</h2>
             <ScoreResult
               result={scoreResult}
               error={scoreError}
@@ -903,7 +906,7 @@
         <div class="options-section">
           <!-- Context Options -->
           <div class="card">
-            <h2 class="card-title">Options</h2>
+            <h2 class="card-title">{$t.options}</h2>
             <ContextOptions
               bind:isTsumo
               bind:isRiichi
@@ -928,9 +931,9 @@
             disabled={!canCalculate || isCalculating}
           >
             {#if isCalculating}
-              Calculating...
+              {$t.calculating}
             {:else}
-              Calculate Score
+              {$t.calculateScore}
             {/if}
           </button>
         </div>
@@ -940,8 +943,8 @@
 
   <footer class="footer">
     <p>
-      Powered by <a href="https://github.com/ryblogs/agari" target="_blank" rel="noopener">Agari</a>
-      — A Riichi Mahjong scoring engine written in Rust
+      {$t.footerPoweredBy} <a href="https://github.com/ryblogs/agari" target="_blank" rel="noopener">Agari</a>
+      {$t.footerDescription}
     </p>
   </footer>
 </div>
diff --git a/web/src/lib/components/ContextOptions.svelte b/web/src/lib/components/ContextOptions.svelte
index 6453825..8a04635 100644
--- a/web/src/lib/components/ContextOptions.svelte
+++ b/web/src/lib/components/ContextOptions.svelte
@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { WIND_NAMES } from '../agari';
+  import { t, locale, getWindNames } from '../i18n';
 
   interface Props {
     isTsumo: boolean;
@@ -43,6 +43,7 @@
 
   const winds = ['east', 'south', 'west', 'north'] as const;
   const windSymbols = { east: '東', south: '南', west: '西', north: '北' };
+  const windNames = $derived(getWindNames($locale));
 
   const handleRiichiChange = () => {
     if (!isRiichi) {
@@ -65,7 +66,7 @@
 <div class="context-options">
   <!-- Win Type -->
   <div class="option-section">
-    <h3 class="section-title">Win Type</h3>
+    <h3 class="section-title">{$t.winType}</h3>
     <div class="toggle-group">
       <button
         type="button"
@@ -73,7 +74,7 @@
         class:active={!isTsumo}
         onclick={() => { isTsumo = false; onChange(); }}
       >
-        Ron
+        {$t.ron}
       </button>
       <button
         type="button"
@@ -81,25 +82,25 @@
         class:active={isTsumo}
         onclick={() => { isTsumo = true; onChange(); }}
       >
-        Tsumo
+        {$t.tsumo}
       </button>
     </div>
   </div>
 
   <!-- Winds -->
   <div class="option-section">
-    <h3 class="section-title">Winds</h3>
+    <h3 class="section-title">{$t.winds}</h3>
     <div class="winds-grid">
       <div class="wind-selector">
-        <label class="wind-label">Round</label>
-        <div class="wind-buttons">
+        <span class="wind-label">{$t.round}</span>
+        <div class="wind-buttons" role="group" aria-label={$t.round}>
           {#each winds as wind}
             <button
               type="button"
               class="wind-btn"
               class:active={roundWind === wind}
               onclick={() => { roundWind = wind; onChange(); }}
-              title={WIND_NAMES[wind]}
+              title={windNames[wind]}
             >
               {windSymbols[wind]}
             </button>
@@ -107,15 +108,15 @@
         </div>
       </div>
       <div class="wind-selector">
-        <label class="wind-label">Seat</label>
-        <div class="wind-buttons">
+        <span class="wind-label">{$t.seat}</span>
+        <div class="wind-buttons" role="group" aria-label={$t.seat}>
           {#each winds as wind}
             <button
               type="button"
               class="wind-btn"
               class:active={seatWind === wind}
               onclick={() => { seatWind = wind; onChange(); }}
-              title={WIND_NAMES[wind]}
+              title={windNames[wind]}
             >
               {windSymbols[wind]}
             </button>
@@ -124,15 +125,15 @@
       </div>
     </div>
     {#if isDealer}
-      <div class="dealer-badge">Dealer (Oya)</div>
+      <div class="dealer-badge">{$t.dealerOya}</div>
     {/if}
   </div>
 
   <!-- Riichi Options -->
   <div class="option-section">
-    <h3 class="section-title">Riichi</h3>
+    <h3 class="section-title">{$t.riichi}</h3>
     {#if hasOpenMelds}
-      <div class="open-hand-notice">Open hand — Riichi not available</div>
+      <div class="open-hand-notice">{$t.openHandNotice}</div>
     {/if}
     <div class="checkbox-group">
       <label class="checkbox-item" class:disabled={hasOpenMelds}>
@@ -142,7 +143,7 @@
           disabled={hasOpenMelds}
           onchange={handleRiichiChange}
         />
-        <span class="checkbox-label">Riichi</span>
+        <span class="checkbox-label">{$t.riichi}</span>
         <span class="han-indicator">+1</span>
       </label>
       <label class="checkbox-item" class:disabled={hasOpenMelds || !isRiichi}>
@@ -152,7 +153,7 @@
           disabled={hasOpenMelds || !isRiichi}
           onchange={handleDoubleRiichiChange}
         />
-        <span class="checkbox-label">Double Riichi</span>
+        <span class="checkbox-label">{$t.doubleRiichi}</span>
         <span class="han-indicator">+1</span>
       </label>
       <label class="checkbox-item" class:disabled={hasOpenMelds || !isRiichi}>
@@ -162,7 +163,7 @@
           disabled={hasOpenMelds || !isRiichi}
           onchange={onChange}
         />
-        <span class="checkbox-label">Ippatsu</span>
+        <span class="checkbox-label">{$t.ippatsu}</span>
         <span class="han-indicator">+1</span>
       </label>
     </div>
@@ -170,12 +171,12 @@
 
   <!-- Situational Yaku -->
   <div class="option-section">
-    <h3 class="section-title">Situational</h3>
+    <h3 class="section-title">{$t.situational}</h3>
     <div class="checkbox-group">
       <label class="checkbox-item">
         <input type="checkbox" bind:checked={isLastTile} onchange={onChange} />
         <span class="checkbox-label">
-          {isTsumo ? 'Haitei (Last Draw)' : 'Houtei (Last Discard)'}
+          {isTsumo ? $t.haitei : $t.houtei}
         </span>
         <span class="han-indicator">+1</span>
       </label>
@@ -186,7 +187,7 @@
           disabled={!isTsumo}
           onchange={onChange}
         />
-        <span class="checkbox-label">Rinshan Kaihou</span>
+        <span class="checkbox-label">{$t.rinshanKaihou}</span>
         <span class="han-indicator">+1</span>
       </label>
       <label class="checkbox-item" class:disabled={isTsumo}>
@@ -196,7 +197,7 @@
           disabled={isTsumo}
           onchange={onChange}
         />
-        <span class="checkbox-label">Chankan</span>
+        <span class="checkbox-label">{$t.chankan}</span>
         <span class="han-indicator">+1</span>
       </label>
     </div>
@@ -204,7 +205,7 @@
 
   <!-- Yakuman Starters -->
   <div class="option-section">
-    <h3 class="section-title">First Turn Yakuman</h3>
+    <h3 class="section-title">{$t.firstTurnYakuman}</h3>
     <div class="checkbox-group">
       <label class="checkbox-item" class:disabled={!isDealer || !isTsumo}>
         <input
@@ -213,7 +214,7 @@
           disabled={!isDealer || !isTsumo}
           onchange={onChange}
         />
-        <span class="checkbox-label">Tenhou</span>
+        <span class="checkbox-label">{$t.tenhou}</span>
         <span class="han-indicator yakuman">役満</span>
       </label>
       <label class="checkbox-item" class:disabled={isDealer || !isTsumo}>
@@ -223,7 +224,7 @@
           disabled={isDealer || !isTsumo}
           onchange={onChange}
         />
-        <span class="checkbox-label">Chiihou</span>
+        <span class="checkbox-label">{$t.chiihou}</span>
         <span class="han-indicator yakuman">役満</span>
       </label>
     </div>
diff --git a/web/src/lib/components/DoraPicker.svelte b/web/src/lib/components/DoraPicker.svelte
index 49af504..6ccefa7 100644
--- a/web/src/lib/components/DoraPicker.svelte
+++ b/web/src/lib/components/DoraPicker.svelte
@@ -1,5 +1,6 @@
 <script lang="ts">
   import { ALL_TILES } from '../agari';
+  import { t } from '../i18n';
   import Tile from './Tile.svelte';
 
   interface Props {
@@ -42,11 +43,11 @@
 
 <svelte:window onkeydown={handleKeydown} />
 
-<!-- svelte-ignore a11y_click_events_have_key_events a11y_interactive_supports_focus -->
-<div class="picker-backdrop" role="dialog" aria-modal="true" onclick={handleBackdropClick}>
+<!-- svelte-ignore a11y_click_events_have_key_events -->
+<div class="picker-backdrop" role="dialog" aria-modal="true" tabindex="-1" onclick={handleBackdropClick}>
   <div class="picker-panel">
     <div class="picker-header">
-      <span>Select Tile</span>
+      <span>{$t.selectTile}</span>
       <button type="button" class="close-btn" onclick={onClose}>×</button>
     </div>
 
diff --git a/web/src/lib/components/LanguageSwitcher.svelte b/web/src/lib/components/LanguageSwitcher.svelte
new file mode 100644
index 0000000..38c4c83
--- /dev/null
+++ b/web/src/lib/components/LanguageSwitcher.svelte
@@ -0,0 +1,65 @@
+<script lang="ts">
+  import { locale, t, availableLocales, type Locale } from '../i18n';
+
+  function handleChange(event: Event) {
+    const select = event.target as HTMLSelectElement;
+    locale.set(select.value as Locale);
+  }
+</script>
+
+<div class="language-switcher">
+  <label class="language-label" for="language-select">
+    {$t.language}
+  </label>
+  <select
+    id="language-select"
+    class="language-select"
+    value={$locale}
+    onchange={handleChange}
+  >
+    {#each availableLocales as loc}
+      <option value={loc.code}>{loc.nativeName}</option>
+    {/each}
+  </select>
+</div>
+
+<style>
+  .language-switcher {
+    display: flex;
+    align-items: center;
+    gap: var(--space-2);
+    margin-left: auto;
+  }
+
+  .language-label {
+    font-size: 0.6875rem;
+    color: var(--text-muted);
+    text-transform: uppercase;
+    letter-spacing: 0.05em;
+  }
+
+  .language-select {
+    padding: var(--space-1) var(--space-2);
+    font-size: 0.8125rem;
+    background: var(--bg-elevated);
+    border: 1px solid var(--border);
+    color: var(--text-primary);
+    cursor: pointer;
+    transition: all 0.15s ease;
+  }
+
+  .language-select:hover {
+    border-color: var(--text-muted);
+  }
+
+  .language-select:focus {
+    outline: none;
+    border-color: var(--accent);
+  }
+
+  @media (max-width: 768px) {
+    .language-label {
+      display: none;
+    }
+  }
+</style>
diff --git a/web/src/lib/components/ScoreResult.svelte b/web/src/lib/components/ScoreResult.svelte
index 32e6689..f989318 100644
--- a/web/src/lib/components/ScoreResult.svelte
+++ b/web/src/lib/components/ScoreResult.svelte
@@ -1,5 +1,25 @@
 <script lang="ts">
   import type { ScoringOutput } from '../agari';
+  import type { Translations } from '../i18n/types';
+  import { t } from '../i18n';
+  import { yakuNameMap, scoreLevelMap } from '../i18n';
+
+  // Reactive translation helpers that depend on $t
+  function translateYaku(backendName: string, trans: Translations): string {
+    const key = yakuNameMap[backendName];
+    if (key) {
+      return trans[key] as string;
+    }
+    return backendName;
+  }
+
+  function translateScoreLevel(backendLevel: string, trans: Translations): string {
+    const key = scoreLevelMap[backendLevel];
+    if (key) {
+      return trans[key] as string;
+    }
+    return backendLevel;
+  }
 
   interface Props {
     result: ScoringOutput | null;
@@ -9,10 +29,10 @@
 
   let { result, error = null, loading = false }: Props = $props();
 
-  const formatPayment = (payment: ScoringOutput['payment'], isDealer: boolean, isTsumo: boolean): string => {
-    if (isTsumo) {
+  const formatPayment = (payment: ScoringOutput['payment'], isDealer: boolean, isTsumoWin: boolean): string => {
+    if (isTsumoWin) {
       if (isDealer) {
-        return `${payment.from_non_dealer?.toLocaleString()} all`;
+        return `${payment.from_non_dealer?.toLocaleString()} ${$t.all}`;
       } else {
         return `${payment.from_non_dealer?.toLocaleString()} / ${payment.from_dealer?.toLocaleString()}`;
       }
@@ -38,7 +58,7 @@
   {#if loading}
     <div class="loading">
       <div class="spinner"></div>
-      <span>Calculating...</span>
+      <span>{$t.calculating}</span>
     </div>
   {:else if error}
     <div class="error">
@@ -48,7 +68,7 @@
     <div class="result-content">
       {#if result.inferred_winning_tile}
         <div class="inferred-warning">
-          Winning tile inferred as <strong>{result.inferred_winning_tile}</strong>
+          {$t.inferredWinningTile} <strong>{result.inferred_winning_tile}</strong>
         </div>
       {/if}
 
@@ -56,44 +76,44 @@
       <div class="score-summary">
         {#if result.score_level}
           <div class="score-level {getScoreLevelClass(result.score_level)}">
-            {result.score_level}
+            {translateScoreLevel(result.score_level, $t)}
           </div>
         {/if}
 
         <div class="score-numbers">
           <div class="han-fu">
             <span class="value">{result.total_han}</span>
-            <span class="unit">han</span>
+            <span class="unit">{$t.han}</span>
           </div>
           <span class="divider">/</span>
           <div class="han-fu">
             <span class="value">{result.fu}</span>
-            <span class="unit">fu</span>
+            <span class="unit">{$t.fu}</span>
           </div>
         </div>
 
         <div class="total-points">
           <span class="points-value">{result.payment.total.toLocaleString()}</span>
-          <span class="points-label">pts</span>
+          <span class="points-label">{$t.pts}</span>
         </div>
 
         <div class="payment-breakdown">
           {formatPayment(result.payment, result.is_dealer, isTsumo)}
           {#if result.is_dealer}
-            <span class="dealer-tag">Dealer</span>
+            <span class="dealer-tag">{$t.dealer}</span>
           {/if}
         </div>
       </div>
 
       <!-- Yaku List -->
       <div class="yaku-section">
-        <h3 class="section-title">Yaku</h3>
+        <h3 class="section-title">{$t.yaku}</h3>
         <div class="yaku-list">
           {#each result.yaku as yaku}
             <div class="yaku-item">
-              <span class="yaku-name">{yaku.name}</span>
+              <span class="yaku-name">{translateYaku(yaku.name, $t)}</span>
               <span class="yaku-han" class:yakuman={yaku.is_yakuman}>
-                {#if yaku.is_yakuman}役満{:else}{yaku.han}{/if}
+                {#if yaku.is_yakuman}{$t.scoreLevelYakuman}{:else}{yaku.han}{/if}
               </span>
             </div>
           {/each}
@@ -103,23 +123,23 @@
       <!-- Dora -->
       {#if result.dora.total > 0}
         <div class="dora-section">
-          <h3 class="section-title">Dora</h3>
+          <h3 class="section-title">{$t.dora}</h3>
           <div class="dora-breakdown">
             {#if result.dora.regular > 0}
               <div class="dora-item">
-                <span>Dora</span>
+                <span>{$t.dora}</span>
                 <span class="dora-count">{result.dora.regular}</span>
               </div>
             {/if}
             {#if result.dora.ura > 0}
               <div class="dora-item">
-                <span>Ura</span>
+                <span>{$t.ura}</span>
                 <span class="dora-count ura">{result.dora.ura}</span>
               </div>
             {/if}
             {#if result.dora.aka > 0}
               <div class="dora-item">
-                <span>Aka</span>
+                <span>{$t.aka}</span>
                 <span class="dora-count aka">{result.dora.aka}</span>
               </div>
             {/if}
@@ -129,44 +149,44 @@
 
       <!-- Fu Breakdown -->
       <details class="fu-details">
-        <summary>Fu Breakdown</summary>
+        <summary>{$t.fuBreakdown}</summary>
         <div class="fu-breakdown">
           <div class="fu-item">
-            <span>Base</span>
+            <span>{$t.fuBase}</span>
             <span>{result.fu_breakdown.base}</span>
           </div>
           {#if result.fu_breakdown.menzen_ron > 0}
             <div class="fu-item">
-              <span>Menzen Ron</span>
+              <span>{$t.fuMenzenRon}</span>
               <span>+{result.fu_breakdown.menzen_ron}</span>
             </div>
           {/if}
           {#if result.fu_breakdown.tsumo > 0}
             <div class="fu-item">
-              <span>Tsumo</span>
+              <span>{$t.fuTsumo}</span>
               <span>+{result.fu_breakdown.tsumo}</span>
             </div>
           {/if}
           {#if result.fu_breakdown.melds > 0}
             <div class="fu-item">
-              <span>Melds</span>
+              <span>{$t.fuMelds}</span>
               <span>+{result.fu_breakdown.melds}</span>
             </div>
           {/if}
           {#if result.fu_breakdown.pair > 0}
             <div class="fu-item">
-              <span>Pair</span>
+              <span>{$t.fuPair}</span>
               <span>+{result.fu_breakdown.pair}</span>
             </div>
           {/if}
           {#if result.fu_breakdown.wait > 0}
             <div class="fu-item">
-              <span>Wait</span>
+              <span>{$t.fuWait}</span>
               <span>+{result.fu_breakdown.wait}</span>
             </div>
           {/if}
           <div class="fu-item total">
-            <span>Total</span>
+            <span>{$t.fuTotal}</span>
             <span>{result.fu_breakdown.raw_total} → {result.fu_breakdown.rounded}</span>
           </div>
         </div>
@@ -174,13 +194,13 @@
 
       <!-- Hand Structure -->
       <div class="structure-section">
-        <span class="structure-label">Structure:</span>
+        <span class="structure-label">{$t.structure}</span>
         <code class="structure-value">{result.hand_structure}</code>
       </div>
     </div>
   {:else}
     <div class="empty-state">
-      <span class="empty-text">Enter a complete hand to calculate score</span>
+      <span class="empty-text">{$t.enterCompleteHand}</span>
     </div>
   {/if}
 </div>
diff --git a/web/src/lib/i18n/en.ts b/web/src/lib/i18n/en.ts
new file mode 100644
index 0000000..02328c2
--- /dev/null
+++ b/web/src/lib/i18n/en.ts
@@ -0,0 +1,172 @@
+import type { Translations } from "./types";
+
+export const en: Translations = {
+  // Header
+  tagline: "Riichi Mahjong Calculator",
+
+  // Loading/Error states
+  loadingCalculator: "Loading calculator...",
+  failedToLoad: "Failed to load calculator:",
+
+  // Hand builder
+  buildYourHand: "Build Your Hand",
+  clear: "Clear",
+  addMeld: "Add Meld:",
+  chi: "Chi",
+  pon: "Pon",
+  openKan: "Open Kan",
+  closedKan: "Closed Kan",
+
+  // Meld builder
+  buildingChi: "Building Chi",
+  buildingPon: "Building Pon",
+  buildingOpenKan: "Building Open Kan",
+  buildingClosedKan: "Building Closed Kan",
+  hintChi: "(select 3 sequential tiles of the same suit)",
+  hintPon: "(select 3 of the same tile)",
+  hintKan: "(select 4 of the same tile)",
+  cancel: "Cancel",
+  confirmAddMeld: "Add Meld",
+
+  // Melds display
+  calledMelds: "Called Melds",
+
+  // Hand display
+  yourHand: "Your Hand",
+  selectWinningTileHint: "Click a tile to select it as winning tile",
+  winBadge: "WIN",
+
+  // Shanten
+  complete: "Complete",
+  tenpai: "Tenpai",
+  shanten: "-shanten",
+
+  // Dora section
+  doraIndicators: "Dora Indicators",
+  dora: "Dora",
+  uraDora: "Ura Dora",
+  addButton: "+ Add",
+  akadoraInHand: "Aka Dora in hand:",
+
+  // Results
+  results: "Results",
+  calculating: "Calculating...",
+  inferredWinningTile: "Winning tile inferred as",
+  han: "han",
+  fu: "fu",
+  pts: "pts",
+  all: "all",
+  dealer: "Dealer",
+  dealerOya: "Dealer (Oya)",
+  yaku: "Yaku",
+  ura: "Ura",
+  aka: "Aka",
+  fuBreakdown: "Fu Breakdown",
+  fuBase: "Base",
+  fuMenzenRon: "Menzen Ron",
+  fuTsumo: "Tsumo",
+  fuMelds: "Melds",
+  fuPair: "Pair",
+  fuWait: "Wait",
+  fuTotal: "Total",
+  structure: "Structure:",
+  enterCompleteHand: "Enter a complete hand to calculate score",
+
+  // Options
+  options: "Options",
+  winType: "Win Type",
+  ron: "Ron",
+  tsumo: "Tsumo",
+  winds: "Winds",
+  round: "Round",
+  seat: "Seat",
+  riichi: "Riichi",
+  openHandNotice: "Open hand — Riichi not available",
+  doubleRiichi: "Double Riichi",
+  ippatsu: "Ippatsu",
+  situational: "Situational",
+  haitei: "Haitei (Last Draw)",
+  houtei: "Houtei (Last Discard)",
+  rinshanKaihou: "Rinshan Kaihou",
+  chankan: "Chankan",
+  firstTurnYakuman: "First Turn Yakuman",
+  tenhou: "Tenhou",
+  chiihou: "Chiihou",
+
+  // Calculate button
+  calculateScore: "Calculate Score",
+
+  // Footer
+  footerPoweredBy: "Powered by",
+  footerDescription: "— A Riichi Mahjong scoring engine written in Rust",
+
+  // Tile picker
+  selectTile: "Select Tile",
+
+  // Wind names
+  windEast: "東 East",
+  windSouth: "南 South",
+  windWest: "西 West",
+  windNorth: "北 North",
+
+  // Language
+  language: "Language",
+
+  // Score levels
+  scoreLevelMangan: "Mangan",
+  scoreLevelHaneman: "Haneman",
+  scoreLevelBaiman: "Baiman",
+  scoreLevelSanbaiman: "Sanbaiman",
+  scoreLevelYakuman: "Yakuman",
+  scoreLevelDoubleYakuman: "Double Yakuman",
+  scoreLevelCountedYakuman: "Counted Yakuman",
+
+  // Yaku names
+  yakuRiichi: "Riichi",
+  yakuIppatsu: "Ippatsu",
+  yakuMenzenTsumo: "Menzen Tsumo",
+  yakuTanyao: "Tanyao (All Simples)",
+  yakuPinfu: "Pinfu",
+  yakuIipeikou: "Iipeikou (Pure Double Sequence)",
+  yakuYakuhaiEast: "Yakuhai: East Wind",
+  yakuYakuhaiSouth: "Yakuhai: South Wind",
+  yakuYakuhaiWest: "Yakuhai: West Wind",
+  yakuYakuhaiNorth: "Yakuhai: North Wind",
+  yakuYakuhaiWhite: "Yakuhai: White Dragon (Haku)",
+  yakuYakuhaiGreen: "Yakuhai: Green Dragon (Hatsu)",
+  yakuYakuhaiRed: "Yakuhai: Red Dragon (Chun)",
+  yakuRinshanKaihou: "Rinshan Kaihou (After Kan)",
+  yakuChankan: "Chankan (Robbing the Kan)",
+  yakuHaitei: "Haitei Raoyue (Last Tile Draw)",
+  yakuHoutei: "Houtei Raoyui (Last Tile Discard)",
+  yakuDoubleRiichi: "Double Riichi",
+  yakuToitoi: "Toitoi (All Triplets)",
+  yakuSanshokuDoujun: "Sanshoku Doujun (Mixed Triple Sequence)",
+  yakuSanshokuDoukou: "Sanshoku Doukou (Triple Triplets)",
+  yakuIttsu: "Ittsu (Pure Straight)",
+  yakuChiitoitsu: "Chiitoitsu (Seven Pairs)",
+  yakuChanta: "Chanta (Outside Hand)",
+  yakuSanAnkou: "San Ankou (Three Concealed Triplets)",
+  yakuSanKantsu: "San Kantsu (Three Kans)",
+  yakuHonroutou: "Honroutou (All Terminals and Honors)",
+  yakuShousangen: "Shousangen (Little Three Dragons)",
+  yakuHonitsu: "Honitsu (Half Flush)",
+  yakuJunchan: "Junchan (Terminals in All Groups)",
+  yakuRyanpeikou: "Ryanpeikou (Twice Pure Double Sequence)",
+  yakuChinitsu: "Chinitsu (Full Flush)",
+  yakuTenhou: "Tenhou (Heavenly Hand)",
+  yakuChiihou: "Chiihou (Earthly Hand)",
+  yakuKokushiMusou: "Kokushi Musou (Thirteen Orphans)",
+  yakuSuuankou: "Suuankou (Four Concealed Triplets)",
+  yakuDaisangen: "Daisangen (Big Three Dragons)",
+  yakuShousuushii: "Shousuushii (Little Four Winds)",
+  yakuDaisuushii: "Daisuushii (Big Four Winds)",
+  yakuTsuuiisou: "Tsuuiisou (All Honors)",
+  yakuChinroutou: "Chinroutou (All Terminals)",
+  yakuRyuuiisou: "Ryuuiisou (All Green)",
+  yakuChuurenPoutou: "Chuuren Poutou (Nine Gates)",
+  yakuKokushi13Wait: "Kokushi Juusanmen (Kokushi Musou 13-wait)",
+  yakuSuuankouTanki: "Suuankou Tanki",
+  yakuJunseiChuurenPoutou: "Junsei Chuuren Poutou",
+  yakuSuuKantsu: "Suu Kantsu (Four Kans)",
+};
diff --git a/web/src/lib/i18n/index.ts b/web/src/lib/i18n/index.ts
new file mode 100644
index 0000000..153e0fb
--- /dev/null
+++ b/web/src/lib/i18n/index.ts
@@ -0,0 +1,191 @@
+/**
+ * Internationalization (i18n) module for Agari WebUI
+ *
+ * Uses Svelte writable store for reactivity and localStorage for persistence.
+ */
+
+import { writable, derived, get } from "svelte/store";
+import type { Locale, Translations } from "./types";
+import { en } from "./en";
+import { ja } from "./ja";
+
+// Mapping from backend yaku names to translation keys
+// These must match exactly what agari-wasm/src/lib.rs yaku_name() returns
+export const yakuNameMap: Record<string, keyof Translations> = {
+  Riichi: "yakuRiichi",
+  Ippatsu: "yakuIppatsu",
+  "Menzen Tsumo": "yakuMenzenTsumo",
+  Tanyao: "yakuTanyao",
+  Pinfu: "yakuPinfu",
+  Iipeikou: "yakuIipeikou",
+  "Yakuhai (East)": "yakuYakuhaiEast",
+  "Yakuhai (South)": "yakuYakuhaiSouth",
+  "Yakuhai (West)": "yakuYakuhaiWest",
+  "Yakuhai (North)": "yakuYakuhaiNorth",
+  "Yakuhai (Haku)": "yakuYakuhaiWhite",
+  "Yakuhai (Hatsu)": "yakuYakuhaiGreen",
+  "Yakuhai (Chun)": "yakuYakuhaiRed",
+  "Rinshan Kaihou": "yakuRinshanKaihou",
+  Chankan: "yakuChankan",
+  "Haitei Raoyue": "yakuHaitei",
+  "Houtei Raoyui": "yakuHoutei",
+  "Double Riichi": "yakuDoubleRiichi",
+  Toitoi: "yakuToitoi",
+  "Sanshoku Doujun": "yakuSanshokuDoujun",
+  "Sanshoku Doukou": "yakuSanshokuDoukou",
+  Ittsu: "yakuIttsu",
+  Chiitoitsu: "yakuChiitoitsu",
+  Chanta: "yakuChanta",
+  "San Ankou": "yakuSanAnkou",
+  "San Kantsu": "yakuSanKantsu",
+  Honroutou: "yakuHonroutou",
+  Shousangen: "yakuShousangen",
+  Honitsu: "yakuHonitsu",
+  Junchan: "yakuJunchan",
+  Ryanpeikou: "yakuRyanpeikou",
+  Chinitsu: "yakuChinitsu",
+  Tenhou: "yakuTenhou",
+  Chiihou: "yakuChiihou",
+  "Kokushi Musou": "yakuKokushiMusou",
+  Suuankou: "yakuSuuankou",
+  Daisangen: "yakuDaisangen",
+  Shousuushii: "yakuShousuushii",
+  Daisuushii: "yakuDaisuushii",
+  Tsuuiisou: "yakuTsuuiisou",
+  Chinroutou: "yakuChinroutou",
+  Ryuuiisou: "yakuRyuuiisou",
+  "Chuuren Poutou": "yakuChuurenPoutou",
+  "Kokushi 13-Wait": "yakuKokushi13Wait",
+  "Suuankou Tanki": "yakuSuuankouTanki",
+  "Junsei Chuuren Poutou": "yakuJunseiChuurenPoutou",
+};
+
+// Mapping from backend score level names to translation keys
+export const scoreLevelMap: Record<string, keyof Translations> = {
+  Mangan: "scoreLevelMangan",
+  Haneman: "scoreLevelHaneman",
+  Baiman: "scoreLevelBaiman",
+  Sanbaiman: "scoreLevelSanbaiman",
+  Yakuman: "scoreLevelYakuman",
+  "Double Yakuman": "scoreLevelDoubleYakuman",
+  "Counted Yakuman": "scoreLevelCountedYakuman",
+};
+
+// Storage key for locale preference
+const LOCALE_STORAGE_KEY = "agari-locale";
+
+// Available translations
+const translations: Record<Locale, Translations> = { en, ja };
+
+// Available locales with display names
+export const availableLocales: {
+  code: Locale;
+  name: string;
+  nativeName: string;
+}[] = [
+  { code: "en", name: "English", nativeName: "English" },
+  { code: "ja", name: "Japanese", nativeName: "日本語" },
+];
+
+/**
+ * Get the initial locale from localStorage or default to 'en'
+ */
+function getInitialLocale(): Locale {
+  if (typeof window === "undefined") return "en";
+
+  const stored = localStorage.getItem(LOCALE_STORAGE_KEY);
+  if (stored && (stored === "en" || stored === "ja")) {
+    return stored;
+  }
+
+  // Optionally detect browser language
+  const browserLang = navigator.language.split("-")[0];
+  if (browserLang === "ja") {
+    return "ja";
+  }
+
+  return "en";
+}
+
+// Create the locale store
+function createLocaleStore() {
+  const { subscribe, set, update } = writable<Locale>(getInitialLocale());
+
+  return {
+    subscribe,
+    set: (newLocale: Locale) => {
+      if (typeof window !== "undefined") {
+        localStorage.setItem(LOCALE_STORAGE_KEY, newLocale);
+      }
+      set(newLocale);
+    },
+    update,
+  };
+}
+
+// Export the locale store
+export const locale = createLocaleStore();
+
+// Derived store for translations
+export const t = derived(locale, ($locale) => translations[$locale]);
+
+// Helper object for non-reactive access (use stores in components instead)
+export const i18n = {
+  get locale(): Locale {
+    return get(locale);
+  },
+  set locale(newLocale: Locale) {
+    locale.set(newLocale);
+  },
+  get t(): Translations {
+    return get(t);
+  },
+};
+
+// Re-export types
+export type { Locale, Translations };
+
+// Helper to get translated wind names
+export function getWindNames(localeCode: Locale = get(locale)) {
+  const trans = translations[localeCode];
+  return {
+    east: trans.windEast,
+    south: trans.windSouth,
+    west: trans.windWest,
+    north: trans.windNorth,
+  } as const;
+}
+
+/**
+ * Translate a yaku name from backend to current locale
+ * @param backendName - The yaku name from the backend
+ * @param trans - The translations object (pass $t for reactivity in Svelte components)
+ */
+export function translateYaku(
+  backendName: string,
+  trans: Translations,
+): string {
+  const key = yakuNameMap[backendName];
+  if (key) {
+    return trans[key] as string;
+  }
+  // Fallback to backend name if no translation found
+  return backendName;
+}
+
+/**
+ * Translate a score level from backend to current locale
+ * @param backendLevel - The score level from the backend
+ * @param trans - The translations object (pass $t for reactivity in Svelte components)
+ */
+export function translateScoreLevel(
+  backendLevel: string,
+  trans: Translations,
+): string {
+  const key = scoreLevelMap[backendLevel];
+  if (key) {
+    return trans[key] as string;
+  }
+  // Fallback to backend level if no translation found
+  return backendLevel;
+}
diff --git a/web/src/lib/i18n/ja.ts b/web/src/lib/i18n/ja.ts
new file mode 100644
index 0000000..cbc3baf
--- /dev/null
+++ b/web/src/lib/i18n/ja.ts
@@ -0,0 +1,172 @@
+import type { Translations } from "./types";
+
+export const ja: Translations = {
+  // Header
+  tagline: "立直麻雀 点数計算機",
+
+  // Loading/Error states
+  loadingCalculator: "計算機を読み込み中...",
+  failedToLoad: "計算機の読み込みに失敗しました:",
+
+  // Hand builder
+  buildYourHand: "手牌を作成",
+  clear: "クリア",
+  addMeld: "副露を追加:",
+  chi: "チー",
+  pon: "ポン",
+  openKan: "明槓",
+  closedKan: "暗槓",
+
+  // Meld builder
+  buildingChi: "チーを作成中",
+  buildingPon: "ポンを作成中",
+  buildingOpenKan: "明槓を作成中",
+  buildingClosedKan: "暗槓を作成中",
+  hintChi: "(同じ色の連続した3枚を選択)",
+  hintPon: "(同じ牌を3枚選択)",
+  hintKan: "(同じ牌を4枚選択)",
+  cancel: "キャンセル",
+  confirmAddMeld: "追加",
+
+  // Melds display
+  calledMelds: "副露",
+
+  // Hand display
+  yourHand: "手牌",
+  selectWinningTileHint: "和了牌をクリックして選択",
+  winBadge: "和了",
+
+  // Shanten
+  complete: "和了形",
+  tenpai: "聴牌",
+  shanten: "向聴",
+
+  // Dora section
+  doraIndicators: "ドラ表示牌",
+  dora: "ドラ",
+  uraDora: "裏ドラ",
+  addButton: "+ 追加",
+  akadoraInHand: "手牌の赤ドラ:",
+
+  // Results
+  results: "結果",
+  calculating: "計算中...",
+  inferredWinningTile: "和了牌を推定:",
+  han: "翻",
+  fu: "符",
+  pts: "点",
+  all: "オール",
+  dealer: "親",
+  dealerOya: "親",
+  yaku: "役",
+  ura: "裏",
+  aka: "赤",
+  fuBreakdown: "符計算",
+  fuBase: "基本",
+  fuMenzenRon: "門前ロン",
+  fuTsumo: "ツモ",
+  fuMelds: "面子",
+  fuPair: "雀頭",
+  fuWait: "待ち",
+  fuTotal: "合計",
+  structure: "手牌構成:",
+  enterCompleteHand: "完成形を入力して点数を計算",
+
+  // Options
+  options: "オプション",
+  winType: "和了方法",
+  ron: "ロン",
+  tsumo: "ツモ",
+  winds: "風",
+  round: "場風",
+  seat: "自風",
+  riichi: "立直",
+  openHandNotice: "副露あり — 立直不可",
+  doubleRiichi: "ダブル立直",
+  ippatsu: "一発",
+  situational: "状況役",
+  haitei: "海底摸月",
+  houtei: "河底撈魚",
+  rinshanKaihou: "嶺上開花",
+  chankan: "槍槓",
+  firstTurnYakuman: "第一巡役満",
+  tenhou: "天和",
+  chiihou: "地和",
+
+  // Calculate button
+  calculateScore: "点数計算",
+
+  // Footer
+  footerPoweredBy: "Powered by",
+  footerDescription: "— Rustで書かれた立直麻雀点数計算エンジン",
+
+  // Tile picker
+  selectTile: "牌を選択",
+
+  // Wind names
+  windEast: "東",
+  windSouth: "南",
+  windWest: "西",
+  windNorth: "北",
+
+  // Language
+  language: "言語",
+
+  // Score levels
+  scoreLevelMangan: "満貫",
+  scoreLevelHaneman: "跳満",
+  scoreLevelBaiman: "倍満",
+  scoreLevelSanbaiman: "三倍満",
+  scoreLevelYakuman: "役満",
+  scoreLevelDoubleYakuman: "ダブル役満",
+  scoreLevelCountedYakuman: "数え役満",
+
+  // Yaku names
+  yakuRiichi: "立直",
+  yakuIppatsu: "一発",
+  yakuMenzenTsumo: "門前清自摸和",
+  yakuTanyao: "断幺九",
+  yakuPinfu: "平和",
+  yakuIipeikou: "一盃口",
+  yakuYakuhaiEast: "役牌: 東",
+  yakuYakuhaiSouth: "役牌: 南",
+  yakuYakuhaiWest: "役牌: 西",
+  yakuYakuhaiNorth: "役牌: 北",
+  yakuYakuhaiWhite: "役牌: 白",
+  yakuYakuhaiGreen: "役牌: 發",
+  yakuYakuhaiRed: "役牌: 中",
+  yakuRinshanKaihou: "嶺上開花",
+  yakuChankan: "槍槓",
+  yakuHaitei: "海底摸月",
+  yakuHoutei: "河底撈魚",
+  yakuDoubleRiichi: "ダブル立直",
+  yakuToitoi: "対々和",
+  yakuSanshokuDoujun: "三色同順",
+  yakuSanshokuDoukou: "三色同刻",
+  yakuIttsu: "一気通貫",
+  yakuChiitoitsu: "七対子",
+  yakuChanta: "混全帯幺九",
+  yakuSanAnkou: "三暗刻",
+  yakuSanKantsu: "三槓子",
+  yakuHonroutou: "混老頭",
+  yakuShousangen: "小三元",
+  yakuHonitsu: "混一色",
+  yakuJunchan: "純全帯幺九",
+  yakuRyanpeikou: "二盃口",
+  yakuChinitsu: "清一色",
+  yakuTenhou: "天和",
+  yakuChiihou: "地和",
+  yakuKokushiMusou: "国士無双",
+  yakuSuuankou: "四暗刻",
+  yakuDaisangen: "大三元",
+  yakuShousuushii: "小四喜",
+  yakuDaisuushii: "大四喜",
+  yakuTsuuiisou: "字一色",
+  yakuChinroutou: "清老頭",
+  yakuRyuuiisou: "緑一色",
+  yakuChuurenPoutou: "九蓮宝燈",
+  yakuKokushi13Wait: "国士無双十三面待ち",
+  yakuSuuankouTanki: "四暗刻単騎",
+  yakuJunseiChuurenPoutou: "純正九蓮宝燈",
+  yakuSuuKantsu: "四槓子",
+};
diff --git a/web/src/lib/i18n/types.ts b/web/src/lib/i18n/types.ts
new file mode 100644
index 0000000..59763d7
--- /dev/null
+++ b/web/src/lib/i18n/types.ts
@@ -0,0 +1,177 @@
+/**
+ * Type definitions for i18n translations
+ * All translation keys are defined here for type safety
+ */
+
+export type Locale = "en" | "ja";
+
+export interface Translations {
+  // Header
+  tagline: string;
+
+  // Loading/Error states
+  loadingCalculator: string;
+  failedToLoad: string;
+
+  // Hand builder
+  buildYourHand: string;
+  clear: string;
+  addMeld: string;
+  chi: string;
+  pon: string;
+  openKan: string;
+  closedKan: string;
+
+  // Meld builder
+  buildingChi: string;
+  buildingPon: string;
+  buildingOpenKan: string;
+  buildingClosedKan: string;
+  hintChi: string;
+  hintPon: string;
+  hintKan: string;
+  cancel: string;
+  confirmAddMeld: string;
+
+  // Melds display
+  calledMelds: string;
+
+  // Hand display
+  yourHand: string;
+  selectWinningTileHint: string;
+  winBadge: string;
+
+  // Shanten
+  complete: string;
+  tenpai: string;
+  shanten: string;
+
+  // Dora section
+  doraIndicators: string;
+  dora: string;
+  uraDora: string;
+  addButton: string;
+  akadoraInHand: string;
+
+  // Results
+  results: string;
+  calculating: string;
+  inferredWinningTile: string;
+  han: string;
+  fu: string;
+  pts: string;
+  all: string;
+  dealer: string;
+  dealerOya: string;
+  yaku: string;
+  ura: string;
+  aka: string;
+  fuBreakdown: string;
+  fuBase: string;
+  fuMenzenRon: string;
+  fuTsumo: string;
+  fuMelds: string;
+  fuPair: string;
+  fuWait: string;
+  fuTotal: string;
+  structure: string;
+  enterCompleteHand: string;
+
+  // Options
+  options: string;
+  winType: string;
+  ron: string;
+  tsumo: string;
+  winds: string;
+  round: string;
+  seat: string;
+  riichi: string;
+  openHandNotice: string;
+  doubleRiichi: string;
+  ippatsu: string;
+  situational: string;
+  haitei: string;
+  houtei: string;
+  rinshanKaihou: string;
+  chankan: string;
+  firstTurnYakuman: string;
+  tenhou: string;
+  chiihou: string;
+
+  // Calculate button
+  calculateScore: string;
+
+  // Footer
+  footerPoweredBy: string;
+  footerDescription: string;
+
+  // Tile picker
+  selectTile: string;
+
+  // Wind names (with kanji for Japanese)
+  windEast: string;
+  windSouth: string;
+  windWest: string;
+  windNorth: string;
+
+  // Language
+  language: string;
+
+  // Score levels
+  scoreLevelMangan: string;
+  scoreLevelHaneman: string;
+  scoreLevelBaiman: string;
+  scoreLevelSanbaiman: string;
+  scoreLevelYakuman: string;
+  scoreLevelDoubleYakuman: string;
+  scoreLevelCountedYakuman: string;
+
+  // Yaku names
+  yakuRiichi: string;
+  yakuIppatsu: string;
+  yakuMenzenTsumo: string;
+  yakuTanyao: string;
+  yakuPinfu: string;
+  yakuIipeikou: string;
+  yakuYakuhaiEast: string;
+  yakuYakuhaiSouth: string;
+  yakuYakuhaiWest: string;
+  yakuYakuhaiNorth: string;
+  yakuYakuhaiWhite: string;
+  yakuYakuhaiGreen: string;
+  yakuYakuhaiRed: string;
+  yakuRinshanKaihou: string;
+  yakuChankan: string;
+  yakuHaitei: string;
+  yakuHoutei: string;
+  yakuDoubleRiichi: string;
+  yakuToitoi: string;
+  yakuSanshokuDoujun: string;
+  yakuSanshokuDoukou: string;
+  yakuIttsu: string;
+  yakuChiitoitsu: string;
+  yakuChanta: string;
+  yakuSanAnkou: string;
+  yakuSanKantsu: string;
+  yakuHonroutou: string;
+  yakuShousangen: string;
+  yakuHonitsu: string;
+  yakuJunchan: string;
+  yakuRyanpeikou: string;
+  yakuChinitsu: string;
+  yakuTenhou: string;
+  yakuChiihou: string;
+  yakuKokushiMusou: string;
+  yakuSuuankou: string;
+  yakuDaisangen: string;
+  yakuShousuushii: string;
+  yakuDaisuushii: string;
+  yakuTsuuiisou: string;
+  yakuChinroutou: string;
+  yakuRyuuiisou: string;
+  yakuChuurenPoutou: string;
+  yakuKokushi13Wait: string;
+  yakuSuuankouTanki: string;
+  yakuJunseiChuurenPoutou: string;
+  yakuSuuKantsu: string;
+}