As an instructor or teaching assistant for a graduate class at a top institution, is it okay to tell students (e.g. in homework feedback) that they need to improve their English? What's the best way to do so without offending or discouraging them?
I'm specifically asking for math/science-based classes where, for instance, students are required to write proofs which involve a lot of "informal" (i.e. using English rather than mathematical notation) proofs and reasoning.
This happens mostly with international students.