diff --git a/frontend/shared/i18n/lobby-loader.ts b/frontend/shared/i18n/lobby-loader.ts
index 70a4ba0..7b4579d 100644
--- a/frontend/shared/i18n/lobby-loader.ts
+++ b/frontend/shared/i18n/lobby-loader.ts
@@ -8,7 +8,7 @@ export const DEFAULT_LOCALE = lobbyCatalog.locales.default as SupportedLocale;
 export const SUPPORTED_LOCALES = lobbyCatalog.locales.supported as readonly SupportedLocale[];
 
 export function normalizeLocale(rawLocale?: string | null): SupportedLocale {
-  const locale = (rawLocale ?? '').trim().toLowerCase();
+  const locale = (rawLocale ?? '').trim().toLowerCase().replace(/_/g, '-');
   if ((SUPPORTED_LOCALES as readonly string[]).includes(locale)) {
     return locale as SupportedLocale;
   }
diff --git a/frontend/tests/lobby-loader.parity.test.ts b/frontend/tests/lobby-loader.parity.test.ts
index 896c1a0..91bd038 100644
--- a/frontend/tests/lobby-loader.parity.test.ts
+++ b/frontend/tests/lobby-loader.parity.test.ts
@@ -16,7 +16,9 @@ describe('shared lobby i18n loader parity', () => {
 
   it('normalizes browser-style locale tags to supported keyspace locales', () => {
     expect(normalizeLocale('da-DK')).toBe('da');
+    expect(normalizeLocale('da_DK')).toBe('da');
     expect(normalizeLocale('en-US')).toBe('en');
+    expect(normalizeLocale('en_US')).toBe('en');
     expect(normalizeLocale('fr-FR')).toBe('en');
   });