Fix: Allow loading of locales like zh_TW

Deactivate the `nonExplicitSupportedLngs` i18next setting (see
https://www.i18next.com/overview/configuration-options#languages-namespaces-resources
for more details) that caused our supported languages needed to contain
only short codes of the languages to be a valid locale. Therefore
locales like `zh_TW` were considered as not supported and the locale did
fall back to `en`.

src/gmp/locale/lang.js

@@ -54,7 +54,7 @@ const I18N_OPTIONS = {
     loadPath: '/locales/{{ns}}-{{lng}}.json', // e.g. /locales/gsa-en.json
   },
   supportedLngs: whitelist,
-  nonExplicitSupportedLngs: true,
+  nonExplicitSupportedLngs: false,
   interpolation: {
     skipOnVariables: false,
   },