Do I understand correctly that the automatic conversion of content files from e.g. default.txt to default.en.txt only happens when you create a new language using the Panel?
This concept does not really work well if you want to check in the language configurations via git and the content is in .gitignore.
The automatic conversion would then not happen and the pages would not find any content. Would it be an idea that in this case the default.txt files could simply work as a fallback content file for the default language and could be converted ad-hoc if necessary?
Unfortunately, the initial creation of languages in the panel is not flexible enough, since cannot set any options like custom URLs for the languages.