翻译:决定语言和注册语言的问题

Currently I have a site that supports English and German where I have English (en) as the fallback language. I'm using the determinePreferredLanguage() method to automatically translate the site and I'm also using the useStaticFilesLoader so each language corresponds to a json file. Additionally I'm using the registerAvailableLanguageKeys method specifying en and de. I understand the nature of this method is to map all variants of a language key to a unified format (ie. en_UK and en_US to en) but is there a way to tell it to map all "non-registered" languages to say, en?Currently I have a site that supports English a