When I was living and studying in Germany, switching between languages mid-sentence was definitely the norm.

Amongst the foreign students (of all nationalities) it happened because the only common languages were German and English - with varying degrees of fluency. If you couldn't think of the word in whichever of the two languages you happened to be speaking at the time, you threw it in in the other language.

Amongst the English-speakers the most common reason for using the German words/phrase instead of the English was that the meaning didn't readily translate or referred to something very specific and as a result had more effect in its original language.