I'm also German and I'm working with German as A-language (French B, English C).
I have the same advice as Aymeric: try to read as much as you can in your mother tongue, listen to the radio, watch television...
If you're in another country, it's quite normal that you have the impression your mother tongue deteriorates, I made the same experience.
What I do from time to time is write some texts in German, it depends on your creativity and your time of course. You can also write speeches, this is what we did at the beginning of our Master studies in Cologne.
I also write down every collocation and every expression that sounds good and would be convenient for a consecutive or a simultaneous interpretation in a certain context.
I am glad to read your post. I was wondering since you study in Germany (I am German A, Engl B, French C), whether you can recommend or know of a good "collocations dictionary" in German?
I have the "Oxford Collocations Dictionary- for students of English", I find it indispensable really, I love it.
But I have been trying to find something similar in German for quite a long time, and I am told that is does not exist.
I wonder if you or anyone else know of such a helpful dictionary.
I of course have a whole long excel table of "Feste Nomen Verb Verbindungen", that I put together when I came across them while reading the papers, but that is just not the same as this Oxford Dictionary.
I would appreciate any responses on this matter.
I'm sure you'll be surprised yourself about the vocabulary you have in your own mother tongue
Good luck to you!