How to respond to "unfair to write English" comments?
For writing proofs, full knowledge of English is not necessary. Most proofs can be written using a single tense. It is sufficient (and often advisable) to keep the structure of sentences very simple; that is very similar to how logic statements are written using math notation. Arguably, this exercise requires students to use only a small and simple subset of English level. Most students are expected to have it by the admission requirements of the course they signed on.