diff --git a/changelog/unreleased/41741 b/changelog/unreleased/41741
new file mode 100644
index 000000000000..09c8670e8f41
--- /dev/null
+++ b/changelog/unreleased/41741
@@ -0,0 +1,26 @@
+Bugfix: Route requests for .php urls through the front controller
+
+Legacy ajax endpoints returned an HTTP 500 with a PHP fatal error
+'Class "OC" not found' instead of a response. Changing the language in the
+personal settings was the most visible case, but the share dialog's email
+autocomplete, public link thumbnails, trashbin previews and the Google Drive
+OAuth flow were affected in the same way.
+
+Those endpoints are legacy routes whose url is also a real script on disk, for
+example /settings/ajax/setlanguage.php. The front controller rewrite rules in
+.htaccess only forwarded a request to index.php when the requested path did not
+exist, so the web server executed such a script directly - that is without the
+bootstrap index.php would have done - and the request died on the first
+statement. The rewrite rules now always route .php urls through the front
+controller, while static assets keep being served directly and the exempted
+entry points (status.php, remote.php, public.php, cron.php, ocs/v1.php,
+ocs/v2.php, core/ajax/update.php) keep bypassing the router.
+
+Existing installations pick up the new rules on the next upgrade, or by running
+'occ maintenance:update:htaccess'. Independently of that, the personal language
+endpoint is now registered as /settings/personal/changelanguage, matching the
+sibling changepassword route, and the personal profile panel builds the url with
+OC.generateUrl() instead of posting to a relative script path.
+
+https://github.com/owncloud/core/issues/41740
+https://github.com/owncloud/core/pull/41741
diff --git a/lib/private/Setup.php b/lib/private/Setup.php
index 0d79276ab7aa..58ee356e003c 100644
--- a/lib/private/Setup.php
+++ b/lib/private/Setup.php
@@ -498,6 +498,12 @@ public static function updateHtaccess(\OCP\IConfig $config): void {
 			$content .= "\n  RewriteRule ^favicon.ico$ core/img/favicon.ico [L]";
 			$content .= "\n  RewriteRule ^core/js/oc.js$ index.php [PT,E=PATH_INFO:$1]";
 			$content .= "\n  RewriteRule ^core/preview.png$ index.php [PT,E=PATH_INFO:$1]";
+			// Requests for .php urls have to reach the front controller as well.
+			// A couple of legacy routes declare a url which is also a real script
+			// on disk - e.g. /settings/ajax/setlanguage.php - and without this the
+			// -f check below lets the web server execute those scripts directly,
+			// that is without the bootstrap index.php would have done.
+			$content .= "\n  RewriteCond %{REQUEST_URI} \\.php$ [OR]";
 			$content .= "\n  RewriteCond %{REQUEST_FILENAME} !-f";
 			$content .= "\n  RewriteCond %{REQUEST_URI} !^$rewriteBaseRe/core/img/favicon\\.ico$";
 			$content .= "\n  RewriteCond %{REQUEST_URI} !^$rewriteBaseRe/robots\\.txt$";
diff --git a/settings/js/panels/profile.js b/settings/js/panels/profile.js
index 6c181a3d5f87..a2c25e7b4dd6 100644
--- a/settings/js/panels/profile.js
+++ b/settings/js/panels/profile.js
@@ -213,7 +213,7 @@ $(document).ready(function () {
 		// Serialize the data
 		var post = $("#languageinput").serialize();
 		// Ajax foo
-		$.post('ajax/setlanguage.php', post, function (data) {
+		$.post(OC.generateUrl('/settings/personal/changelanguage'), post, function (data) {
 			if (data.status === "success") {
 				location.reload();
 			}
diff --git a/settings/routes.php b/settings/routes.php
index 21868b4f339f..cbe06da50da8 100644
--- a/settings/routes.php
+++ b/settings/routes.php
@@ -104,6 +104,12 @@
 $this->create('settings_personal_changepassword', '/settings/personal/changepassword')
 	->post()
 	->action('OC\Settings\ChangePassword\Controller', 'changePersonalPassword');
+$this->create('settings_personal_changelanguage', '/settings/personal/changelanguage')
+	->post()
+	->actionInclude('settings/ajax/setlanguage.php');
+// Kept for backwards compatibility only. This url is shadowed by the script of
+// the same name on disk, so the front controller rewrite never reaches it - see
+// settings_personal_changelanguage above.
 $this->create('settings_ajax_setlanguage', '/settings/ajax/setlanguage.php')
 	->actionInclude('settings/ajax/setlanguage.php');
 // apps
diff --git a/tests/lib/Route/RouteShadowingTest.php b/tests/lib/Route/RouteShadowingTest.php
new file mode 100644
index 000000000000..e1de1fed64ec
--- /dev/null
+++ b/tests/lib/Route/RouteShadowingTest.php
@@ -0,0 +1,92 @@
+<?php
+/**
+ * @author Thomas Müller <thomas.mueller@tmit.eu>
+ *
+ * @copyright Copyright (c) 2026, ownCloud GmbH
+ * @license AGPL-3.0
+ *
+ * This code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License, version 3,
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ *
+ */
+
+namespace Test\Route;
+
+/**
+ * The front controller rewrite in .htaccess only forwards a request to
+ * index.php when the requested path does NOT exist on disk:
+ *
+ *   RewriteCond %{REQUEST_FILENAME} !-f
+ *   RewriteRule . index.php [PT,E=PATH_INFO:$1]
+ *
+ * A route whose url maps onto a real .php file is therefore unreachable
+ * through the router - the web server executes that script directly, without
+ * the bootstrap that base.php/index.php would have done, and the request dies
+ * with 'Class "OC" not found'. See the personal language endpoint, which used
+ * to be posted to as /settings/ajax/setlanguage.php.
+ *
+ * This guards the endpoints the web ui posts to against regressing into that
+ * shape again.
+ */
+class RouteShadowingTest extends \Test\TestCase {
+	/**
+	 * Urls the js of the web ui posts to, which therefore have to survive the
+	 * front controller rewrite.
+	 */
+	public function providesUiEndpoints() {
+		return [
+			'personal language' => ['/settings/personal/changelanguage'],
+			'personal password' => ['/settings/personal/changepassword'],
+		];
+	}
+
+	/**
+	 * @dataProvider providesUiEndpoints
+	 * @param string $url
+	 */
+	public function testUiEndpointIsNotShadowedByAFileOnDisk($url) {
+		$path = \OC::$SERVERROOT . '/' . \ltrim($url, '/');
+
+		$this->assertFileDoesNotExist(
+			$path,
+			"The route $url is shadowed by a real file on disk. The front " .
+			'controller rewrite skips existing files, so this endpoint would ' .
+			'be executed without a bootstrap and fail with "Class OC not found".'
+		);
+	}
+
+	public function testPersonalLanguageRouteIsRegistered() {
+		$routes = \file_get_contents(\OC::$SERVERROOT . '/settings/routes.php');
+
+		$this->assertStringContainsString(
+			'/settings/personal/changelanguage',
+			$routes,
+			'The personal language endpoint must be routed through the front controller.'
+		);
+	}
+
+	public function testPersonalProfileJsDoesNotPostToARelativeScriptPath() {
+		$js = \file_get_contents(\OC::$SERVERROOT . '/settings/js/panels/profile.js');
+
+		$this->assertStringNotContainsString(
+			"'ajax/setlanguage.php'",
+			$js,
+			'The language selector must build its url with OC.generateUrl() so ' .
+			'the request is routed instead of hitting the script on disk.'
+		);
+		$this->assertStringContainsString(
+			"OC.generateUrl('/settings/personal/changelanguage')",
+			$js,
+			'The language selector must post to the routed endpoint.'
+		);
+	}
+}
diff --git a/tests/lib/SetupTest.php b/tests/lib/SetupTest.php
index 77625549bd5d..f8bbed907721 100644
--- a/tests/lib/SetupTest.php
+++ b/tests/lib/SetupTest.php
@@ -218,7 +218,7 @@ public function testUpdateHtaccess(): void {
 		);
 	}
 
-	public function testUpdateHtaccessWithRewriteBaseUsesFileExistenceCheck(): void {
+	public function testUpdateHtaccessWithRewriteBaseUsesFileExistenceCheckAndRoutesPhpUrls(): void {
 		$origServerRoot = \OC::$SERVERROOT;
 		$htaccessFile = \OC::$SERVERROOT . '/tests/data/.htaccess';
 		\touch($htaccessFile);
@@ -256,5 +256,14 @@ public function testUpdateHtaccessWithRewriteBaseUsesFileExistenceCheck(): void
 			'REQUEST_URI} !\.(css|js|svg|gif|png|html|ttf|woff|ico|jpg|jpeg|json|properties)',
 			$content
 		);
+		// .php urls have to be routed even when the script exists on disk,
+		// otherwise the web server runs it without a bootstrap
+		$phpCond = 'RewriteCond %{REQUEST_URI} \.php$ [OR]';
+		$this->assertStringContainsString($phpCond, $content);
+		$this->assertLessThan(
+			\strpos($content, 'RewriteCond %{REQUEST_FILENAME} !-f'),
+			\strpos($content, $phpCond),
+			'The .php condition has to precede the file existence check to be OR-ed with it'
+		);
 	}
 }