Hi Florian, how are you? I want to help you with few notes :)
You said:
مرحبا يا اصدقائي! من يحبون التعلم ألماني؟
And its understandable but you should say :
مرحباً يا أصدقائي! من يحب تعلم الألمانية؟
you are asking if some "one" wants to learn German.
So you should use the singular verb "يحب"
Not the plural "يحبون"
also, we use " ألماني" when we are talking about a "masculine" adjective.
Like:
He is German → هو ألماني
This is a German book → هذا كتاب ألماني
But in "feminine" adjectives we use "ألمانية"
In your sentence above you said :
من يحبون التعلم ألماني؟
Here you are talking about the language "اللغة" right?
And "اللغة " is feminine so you should say "الألمانية"
Your Arabic is wonderful and you look advanced to me :))