Should I include non-English textbooks in the course?
I think you can distinguish between required and optional materials

For required materials (students will be expected to read regularly, have exams on the content, solve exercises from the book, etc), I'd say they should by default be in the "working language" of the university, unless there is a clear educational reason to do otherwise (e.g. the course is studying a foreign language).  In the latter case, students should be advised before signing up for the course that knowledge of the other language is a requirement.

For optional materials, I see no problem with offering materials in a variety of languages, either for the convenience of the students, or simply because it's a good book that happens to be in another language.  Those who know the language can benefit, and those who don't still have everything they need to keep up with the course.