Current in-app i18n process handles empty keys poorly

[CosmoMyzrailGorynych]

The problem:

• If a key is absent in the selected translation file, its value will display as undefined. Instead, it should always fallback to English language.
• If a key's value is an empty string or any onther false-ish variable, it displays as is. It should instead fallback to English language.
• Existing language files should clear their untranslated keys (rn these are values copied from English.json), so that they become manageable through integrated translation packages (e.g. JSONBabel) (see Existing language files should clear their untranslated keys #97).