fix(i18n): detect Polish and derive language routing from supportedLanguages

Polish (pl) was missing from the hand-maintained language regexes, so
getLanguageFromUrl() returned en for /pl/... pages and the UI fell back
to English after the initial server-rendered Polish content.

Derive the pattern from supportedLanguages so new locales cannot be
missed again, and cover Polish in getLanguageFromUrl(), changeLanguage()
and rewriteLinks() with regression tests.

Refs #972

src/js/i18n/i18n.ts

@@ -61,6 +61,14 @@ export const languageNames: Record<SupportedLanguage, string> = {
   fa: 'فارسی',
 };
 
+// Longest codes first so multi-part tags like zh-TW win over their prefix (zh).
+const LANGUAGE_PATTERN = [...supportedLanguages]
+  .sort((a, b) => b.length - a.length)
+  .join('|');
+
+const URL_LANGUAGE_REGEX = new RegExp(`^/(${LANGUAGE_PATTERN})(?:/|$)`);
+const LANGUAGE_PREFIX_REGEX = new RegExp(`^/(${LANGUAGE_PATTERN})(/.*)?$`);
+
 export const getLanguageFromUrl = (): SupportedLanguage => {
   const basePath = import.meta.env.BASE_URL.replace(/\/$/, '');
   let path = window.location.pathname;
@@ -73,9 +81,7 @@ export const getLanguageFromUrl = (): SupportedLanguage => {
     path = '/' + path;
   }
 
-  const langMatch = path.match(
-    /^\/(en|ar|fr|es|de|zh|zh-TW|vi|tr|id|it|pt|nl|be|da|ko|sv|ru|ja|uk|sk|ca|fa)(?:\/|$)/
-  );
+  const langMatch = path.match(URL_LANGUAGE_REGEX);
   if (
     langMatch &&
     supportedLanguages.includes(langMatch[1] as SupportedLanguage)
@@ -211,9 +217,7 @@ export const changeLanguage = (lang: SupportedLanguage): void => {
   }
 
   let pagePathWithoutLang = relativePath;
-  const langPrefixMatch = relativePath.match(
-    /^\/(en|ar|fr|es|de|zh|zh-TW|vi|tr|id|it|pt|nl|be|da|ko|sv|ru|ja|uk|sk|ca|fa)(\/.*)?$/
-  );
+  const langPrefixMatch = relativePath.match(LANGUAGE_PREFIX_REGEX);
   if (langPrefixMatch) {
     pagePathWithoutLang = langPrefixMatch[2] || '/';
   }
@@ -320,7 +324,7 @@ export const rewriteLinks = (): void => {
     }
 
     const langPrefixRegex = new RegExp(
-      `^(${basePath.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')})?/?(en|ar|fr|es|de|zh|zh-TW|vi|tr|id|it|pt|nl|be|da|ko|sv|ru|ja|uk|sk|ca|fa)(/|$|[?#])`
+      `^(${basePath.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')})?/?(${LANGUAGE_PATTERN})(/|$|[?#])`
     );
     if (langPrefixRegex.test(href)) {
       return;

src/tests/i18n.test.ts

@@ -3,6 +3,8 @@ import {
   changeLanguage,
   getLanguageFromUrl,
   LANGUAGE_CHOICE_KEY,
+  rewriteLinks,
+  supportedLanguages,
 } from '@/js/i18n/i18n';
 
 describe('getLanguageFromUrl', () => {
@@ -75,6 +77,22 @@ describe('getLanguageFromUrl', () => {
     window.location.pathname = '/zh-TW/merge-pdf';
     expect(getLanguageFromUrl()).toBe('zh-TW');
   });
+
+  it('should match every supported language from the URL', () => {
+    supportedLanguages.forEach((lang) => {
+      window.location.pathname = `/${lang}/`;
+      expect(getLanguageFromUrl()).toBe(lang);
+      window.location.pathname = `/${lang}/merge-pdf`;
+      expect(getLanguageFromUrl()).toBe(lang);
+    });
+  });
+
+  it('should match Polish from the URL', () => {
+    window.location.pathname = '/pl/';
+    expect(getLanguageFromUrl()).toBe('pl');
+    window.location.pathname = '/pl/merge-pdf';
+    expect(getLanguageFromUrl()).toBe('pl');
+  });
 });
 
 describe('changeLanguage', () => {
@@ -143,4 +161,52 @@ describe('changeLanguage', () => {
     changeLanguage('ja');
     expect(target).toBe('/pdf/ja/merge-pdf');
   });
+
+  it('swaps the Polish prefix instead of nesting it', () => {
+    at('/pl/merge-pdf');
+    changeLanguage('de');
+    expect(target).toBe('/de/merge-pdf');
+    at('/pl/');
+    changeLanguage('en');
+    expect(target).toBe('/');
+  });
+});
+
+describe('rewriteLinks', () => {
+  const originalLocation = window.location;
+
+  function at(pathname: string) {
+    Object.defineProperty(window, 'location', {
+      value: { ...originalLocation, pathname },
+      writable: true,
+      configurable: true,
+    });
+  }
+
+  beforeEach(() => {
+    document.body.innerHTML = '';
+    vi.stubEnv('BASE_URL', '/');
+  });
+
+  afterEach(() => {
+    Object.defineProperty(window, 'location', {
+      value: originalLocation,
+      writable: true,
+      configurable: true,
+    });
+    document.body.innerHTML = '';
+    vi.unstubAllEnvs();
+  });
+
+  it('prefixes links on Polish pages and leaves language links alone', () => {
+    at('/pl/merge-pdf');
+    document.body.innerHTML =
+      '<a href="/tools">tools</a><a href="/de/merge-pdf">de</a>';
+
+    rewriteLinks();
+
+    const links = document.querySelectorAll('a');
+    expect(links[0].getAttribute('href')).toBe('/pl/tools');
+    expect(links[1].getAttribute('href')).toBe('/de/merge-pdf');
+  });
 });