Sure, I could add de_DE as an additional German language, but please be advised that this solution is suboptimal and will be very frustrating for translators.
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There is no way to tell translators that
deuses informal andde_DEuses formal language. -
The source language will be English. You need to start from scratch. Sure, you could import the existing German translations as a starting point, but afterwards there’s no way to sync them automatically. And as far as I know
deandde_DEdo not share the same translation memory, so that’s no help either. -
If you need the source language to be German you could create your own project on Transifex, add the German translations as source files and optionally configure automatic updates for source files.
I’ll be sending a feature request to Transifex, but honestly I’d be surprised if this will be improved in the near future. Other platforms like Crowdin do a better job by allowing custom languages.