Hi Tela,
Yes this is not translated yet for German. You can language override this language constant if you want to translate it on your site.

MOD_SUBSCRIBE_MESSAGE_CATEGORY="Subscribe to this category"

I would prefer to use transiflex so that it comes into the language file and can be use for everyone instead of hard coding that only for me. So I checked transiflex and it looks like you have a German translation here. So when is that taken into the real language file? Can I help here somehow?

Hey Tela,
Unfortunately, our German translation team is currently full.
Perhaps you can apply here first to the translation team. Once there is a vacant spot, we will add you in.

I joined already (but with a different name, I use BastianWie there). I can see that 100% is translated but only 90% is reviewed. Some of the translated elements seamed to be older then 4 years but still not reviewed. Having a german translation is VERY important for me. I couldn´t handle many thousand german language overwrite strings by hand ...

I´m very willing to spend here some time to get a proper full translation.

We will very appreciate that you willing to help on this German translation, for now when you translated those English to German language, you can put into 1 file then you can upload on our forums, so we will assist you on this to update those German translation on our language server.