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/base/IsoLanguages.ts

@@ -35,11 +35,11 @@ export const IsoLanguages: { [languageCode in I18nResourceCode]: ISOLanguage } &
     },
     'pt-br': PT_BR,
     'pt-BR': PT_BR,
-    zh_Hans: {
+    'zh-Hans': {
         name: 'Chinese (Simplified)',
         nativeName: '中文 (HANS)',
     },
-    zh_Hant: {
+    'zh-Hant': {
         name: 'Chinese (Traditional)',
         nativeName: '中文 (HANT)',
     },