Use BCP-47 language codes for i18n resource files

Lingui expects the codes to be compliant to BCP-47 while weblate was not set up to create the resource files in that format.
And react-i18next was able to just handle them.

fixes #1058

src/i18n/index.ts

@@ -34,8 +34,8 @@ export const i18nResources = [
     'ta',
     'tr',
     'vi',
-    'zh_Hans',
-    'zh_Hant',
+    'zh-Hans',
+    'zh-Hant',
 ] as const;
 
 export type I18nResourceCode = (typeof i18nResources)[number];