Can I put a complete German translation of aour mod into my mod to publishing it?

You don't have to worry about the German translation, because my mod has it then.

Please feel free to write me. Thank you.

The mod is published under a generally permissive license. The main mod page includes a link to that license. Basically you can use the mod however you wish, but must license your mod under the same or more open license.

I am not entirely sure on what you are asking.
If it is to just duplicate the mod and add German translations why not just add them to this version?
If it is to add the mod to a merged mod pack and add German translations to them all then I see no issue as long as you honorar the licence and preferably give credit

My mod is simply a language pack or a mod that only adds the German language. If now your mod is installed, then all is already translated to German, without you having to translate your mod to German.

The mod itself is not changed, except the language.

To add translations should just be adding a new localle file in to this mod.
If there is anywhere I don't support localle that's an oversight and I will correct.

I know, but it's also good to separate the mod from the language files. I also think it's good if a German translates a mod into German instead of a fashion developer who doesn't speak German, for example. Right?

My understanding is its common for native speakers to make the localle file in their language if they wish and pass to the mod maker to include. Having separate versions of mods for each language isn't normal and isn't usual software development practice.

But its possible and easier for the user...

But then you'd have 10 versions of a mod on the portal each in their own language and all potentially on different code versions, depending on when each was last updated. They would also be incompatible with each other in an MP server or if save was shared.

But as said you can create a German clone within the license. It's just not the recommended way to do it.