Hi and welcome. I see that you've started well with translations!
About Fixt, things are not that simple, let me clarify:
1. The translation in the system is an "unofficial" one. It's connected to
this repository.
2. There's an official translation
repo, but as you can see, it's not been updated for years - unfortunately Sduibek hasn't been very active.
3. If the "official" translation gets changed at some point, it'll diverge from the BGforge version, with no simple way to merge them back.
The best option to solve this would be to connect the official repo to the system. However, you'd have to contact Sduibek and convince him to do that.
Alternatively, I can add German to the unofficial repo, if you provide 2 full sets of files - English and German - their versions must correspond 1-1 to each other for import to go successfully. But to reiterate, I don't know what happens if the official one becomes active again.
Also, not sure if you're aware, but there's another project called
F1in2, which is much more active at this point. And they do maintain a German translation - but manually. If your interest is not Fixt specifically, but F1 in general, it might be more worthwhile to spend effort on.
The same applies, though - their repo is not connected to BGforge system, and to connect, you'd have to contact the developers and convince them to do that.