I am basically looking for two words which are, though, related to some degree. It might even be that ultimately the same word is the answer to both parts of this question.
I split them up into two questions. That's the related question.
Anyhow, here's what I'm trying to give it a name.
My mother-tongue is German. I, obviously, speak German very fluently and can convey any kind of ideas. However, if I speak in English for a while and then have to translate something into German, I sometimes fail to come up with German phrases. In that very moment, only English idioms come to my mind. Under different circumstances I certainly could say it in German, but right in that moment I feel like German would be my second language.
Is there a technical term for the inability to express certain things with words or phrases which you actually know by heart and that you would use off-the-cuff under normal circumstances?