diff --git a/web/src/lib/i18n.spec.ts b/web/src/lib/i18n.spec.ts
index 43eabd5e22..ccc60a835b 100644
--- a/web/src/lib/i18n.spec.ts
+++ b/web/src/lib/i18n.spec.ts
@@ -50,5 +50,13 @@ describe('i18n', () => {
       expect(getClosestAvailableLocale(['zh'], allLocales)).toBeUndefined();
       expect(getClosestAvailableLocale(['de', 'zh', 'en-US'], allLocales)).toBe('en-US');
     });
+
+    it('matches underscore-based stored locale codes against normalized locale lists', () => {
+      const allLocales = ['de-CH', 'pt-BR', 'sr-Cyrl', 'zh-Hant'];
+      expect(getClosestAvailableLocale(['de_CH'], allLocales)).toBe('de_CH');
+      expect(getClosestAvailableLocale(['pt_BR'], allLocales)).toBe('pt_BR');
+      expect(getClosestAvailableLocale(['sr_Cyrl'], allLocales)).toBe('sr_Cyrl');
+      expect(getClosestAvailableLocale(['zh_Hant'], allLocales)).toBe('zh_Hant');
+    });
   });
 });
diff --git a/web/src/lib/utils/i18n.ts b/web/src/lib/utils/i18n.ts
index ca7b091774..458d1762ae 100644
--- a/web/src/lib/utils/i18n.ts
+++ b/web/src/lib/utils/i18n.ts
@@ -19,20 +19,20 @@ const fileCodes = Object.keys(modules)
   .map((path) => path.match(/\/(\w+)\.json$/)?.[1])
   .filter(Boolean) as string[];
 
-const convertBCP47 = (code: string) => code.replace('_', '-');
+const convertBCP47 = (code: string) => code.replaceAll('_', '-');
 
 export const langCodes = fileCodes.map((code) => convertBCP47(code));
 
 // https://github.com/kaisermann/svelte-i18n/blob/780932a3e1270d521d348aac8ba03be9df309f04/src/runtime/stores/locale.ts#L11
 const getSubLocales = (locale: string) => {
-  return locale
+  return convertBCP47(locale)
     .split('-')
     .map((_, i, arr) => arr.slice(0, i + 1).join('-'))
     .reverse();
 };
 
 export const getClosestAvailableLocale = (locales: readonly string[], allLocales: readonly string[]) => {
-  const allLocalesSet = new Set(allLocales);
+  const allLocalesSet = new Set(allLocales.map((locale) => convertBCP47(locale)));
   return locales.find((locale) => getSubLocales(locale).some((subLocale) => allLocalesSet.has(subLocale)));
 };