We had good foreign players over the years, some learned little German or were super slow. Heck even some coaches did never learn much (Trappatoni for one; and I wonder how Ancelottis German is
For some like Hernandez, Luca Toni or Ribery it didn't affect their performance or team integration. Ribery being a good example, as long as he thought we were his stepping stone to climb to Real, he learned no German at all. Over time his German is ok now and the question is anyway, what he is a fast learner in, except football (according to my french colleagues, his french language skills are mediocre as well
he took Munich as his residence now, as did some others, like Sagnol, Elber. even Pep learned some German and still owns an Appartment in town.
Given that any top level team is a melting pot of nationalities, and players sometimes stay for only one year or two so what do you expect. Tuchel has worked in France and England, I imagine it is not a big deal. Coaching will sound quite babylonic anyway.