diff --git a/.github/workflows/deploy.yaml b/.github/workflows/deploy.yaml
index 6a93ff78..e5f0ea55 100644
--- a/.github/workflows/deploy.yaml
+++ b/.github/workflows/deploy.yaml
@@ -25,6 +25,8 @@ jobs:
       - name: Build
         if: success()
         run: npm run build
+      - name: Check demo language files
+        run: test -f dist/demo/assets/lang/en-US.lang && test -f dist/demo/assets/lang/de-DE.lang
       - name: Deploy - https://draggable.github.io/formeo/
         if: success()
         uses: peaceiris/actions-gh-pages@v4
diff --git a/.github/workflows/publish.yaml b/.github/workflows/publish.yaml
index 931a75b0..df16f6fe 100644
--- a/.github/workflows/publish.yaml
+++ b/.github/workflows/publish.yaml
@@ -27,6 +27,8 @@ jobs:
         run: npm test
       - name: Build
         run: npm run build
+      - name: Check demo language files
+        run: test -f dist/demo/assets/lang/en-US.lang && test -f dist/demo/assets/lang/de-DE.lang
       - name: Test built package
         run: npm run test:dist
       - name: Publish
diff --git a/src/lib/js/copy-assets.test.mjs b/src/lib/js/copy-assets.test.mjs
new file mode 100644
index 00000000..f3be263e
--- /dev/null
+++ b/src/lib/js/copy-assets.test.mjs
@@ -0,0 +1,53 @@
+import { mkdirSync, mkdtempSync, readdirSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { suite, test } from 'node:test'
+import { copyTargets, targets } from '../../../tools/copy-assets.mjs'
+
+const fixture = (t, files = {}) => {
+  const dir = mkdtempSync(join(tmpdir(), 'formeo-copy-assets-'))
+  t.after(() => rmSync(dir, { recursive: true, force: true }))
+  for (const [path, text] of Object.entries(files)) {
+    mkdirSync(join(dir, path, '..'), { recursive: true })
+    writeFileSync(join(dir, path), text)
+  }
+  return dir
+}
+
+suite('copy-assets', () => {
+  test('copies nested files flat by basename and skips directories', async t => {
+    const src = fixture(t, { 'lang/de-DE.lang': 'de', 'lang/de-DE.json': '{}', 'top.lang': 'top' })
+    const dest = join(fixture(t), 'out')
+
+    t.assert.strictEqual(await copyTargets([{ src: join(src, '**/*'), dest }]), 0)
+    t.assert.deepStrictEqual(readdirSync(dest).sort(), ['de-DE.json', 'de-DE.lang', 'top.lang'])
+  })
+
+  test('the language target copies the installed package files into the demo', async t => {
+    const langTarget = targets.find(({ dest }) => dest.endsWith(join('assets', 'lang')))
+    const dest = fixture(t)
+
+    t.assert.strictEqual(await copyTargets([{ ...langTarget, dest }]), 0)
+    const files = readdirSync(dest)
+    t.assert.ok(files.includes('en-US.lang'))
+    t.assert.ok(files.includes('de-DE.lang'))
+    t.assert.ok(!files.includes('lang'))
+  })
+
+  test('*.js does not pick up .cjs files', async t => {
+    const src = fixture(t, { 'formeo.es.js': '', 'formeo.cjs': '' })
+    const dest = join(fixture(t), 'out')
+
+    await copyTargets([{ src: join(src, '*.js'), dest }])
+    t.assert.deepStrictEqual(readdirSync(dest), ['formeo.es.js'])
+  })
+
+  test('counts a failed copy instead of swallowing it', async t => {
+    t.mock.method(console, 'error', () => {})
+    const src = fixture(t, { 'a.lang': 'a' })
+    const dest = join(fixture(t, { 'not-a-dir': '' }), 'not-a-dir')
+
+    t.assert.strictEqual(await copyTargets([{ src: join(src, '*'), dest }]), 1)
+    t.assert.strictEqual(console.error.mock.callCount(), 1)
+  })
+})
diff --git a/tools/copy-assets.mjs b/tools/copy-assets.mjs
index 3652e5a7..e536eb36 100644
--- a/tools/copy-assets.mjs
+++ b/tools/copy-assets.mjs
@@ -1,13 +1,13 @@
 // copyDir.mjs
 import { promises as fs } from 'node:fs'
 import { basename, dirname, join, resolve } from 'node:path'
-import { fileURLToPath } from 'node:url'
+import { fileURLToPath, pathToFileURL } from 'node:url'
 
 const __dirname = dirname(fileURLToPath(import.meta.url))
 
 const projectRoot = resolve(__dirname, '..')
 
-const targets = [
+export const targets = [
   {
     src: resolve(projectRoot, 'src/lib/icons/formeo-sprite.svg'),
     dest: resolve(projectRoot, 'dist/demo/assets/img/'),
@@ -17,7 +17,8 @@ const targets = [
     dest: resolve(projectRoot, 'dist/'),
   },
   {
-    src: resolve(projectRoot, 'node_modules', '@draggable/formeo-languages/dist/lang/*'),
+    // Any depth: since 3.5.1 the package nests its files in dist/lang/lang/. They are copied flat by basename.
+    src: resolve(projectRoot, 'node_modules', '@draggable/formeo-languages/dist/lang/**/*'),
     dest: resolve(projectRoot, 'dist/demo/assets/lang'),
   },
   {
@@ -37,22 +38,29 @@ const targets = [
 
 async function copyFile(src, dest, rename = null) {
   for await (const file of fs.glob(src)) {
+    if ((await fs.stat(file)).isDirectory()) continue
     const destPath = rename ? join(dest, rename) : join(dest, basename(file))
     await fs.mkdir(dirname(destPath), { recursive: true })
     await fs.copyFile(file, destPath)
   }
 }
 
-// Entry point
-async function main() {
+/** Copy every target, carrying on past errors; returns the number of targets that failed. */
+export async function copyTargets(targets) {
+  let failures = 0
   for (const target of targets) {
     try {
       await copyFile(target.src, target.dest, target.rename)
       console.log(`Copied ${basename(target.src)} to ${target.dest}`)
     } catch (error) {
+      failures++
       console.error(`Error copying file: ${error.message}`)
     }
   }
+  return failures
 }
 
-main()
+if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href) {
+  // A failed copy fails the build, so a broken demo can't be deployed silently.
+  if (await copyTargets(targets)) process.exitCode = 1
+}