I first would recommend that you talk to the writing support services people at your institution, such as the writing center or the center for teaching and learning. They will have some suggestions for you about how to approach this topic in a way that is constructive and useful for students. I'm not in math, but I do have students who have challenges with standard written English, both immigrant and native born.

Here are some ideas I would suggest.

1. I always say that one of my learning outcomes is that you will improve your skill in writing. This is not a grammar class, and writing mechanics will not be an important part of the grading, but what will be important is that you get a lot of practice in professional writing in our discipline.

2. You can also add oral expression to this, and your example of explaining on a walk is a nice way of expressing this. I would even consider adding something about that in your learning outcomes.

3. All the writing pedagogy people will tell you two things. One is that to get better at writing you have to write a lot. The second is that revision is essential. Create opportunities for your students to revise their writing.

4. Don't try to teach a grammar or composition course. You're not actually qualified for that besides the fact that you have math content to cover. If you can help students make small steps in their writing be happy with that.

5. Make sure you encourage students to take advantage of tutoring and other academic supports. Remind them every week that this is an option so that using them becomes de-stigmatized.

Since you are a fellow CUNY person I would strongly suggest that you see if you can get a WAC fellow to work with you. They are generally excellent and have good training.

I first would recommend that you talk to the writing support services people at your institution, such as the writing center or the center for teaching and learning. They will have some suggestions for you about how to approach this topic in a way that is constructive and useful for students. I'm not in math, but I do have students who have challenges with standard written English, both immigrant and native born.

Here are some ideas I would suggest.

1. I always say that one of my learning outcomes is that you will improve your skill in writing. This is not a grammar class, and writing mechanics will not be an important part of the grading, but what will be important is that you get a lot of practice in professional writing in our discipline.

2. You can also add oral expression to this, and your example of explaining on a walk is a nice way of expressing this. I would even consider adding something about that in your learning outcomes.

3. All the writing pedagogy people will tell you two things. One is that to get better at writing you have to write a lot. The second is that revision is essential. Create opportunities for your students to revise their writing.

4. Don't try to teach a grammar or composition course. You're not actually qualified for that besides the fact that you have math content to cover. If you can help students make small steps in their writing be happy with that.

5. Make sure you encourage students to take advantage of tutoring and other academic supports. Remind them every week that this is an option so that using them becomes de-stigmatized.

Since you are a fellow CUNY person I would strongly suggest that you see if you can get a WAC fellow to work with you. They are generally excellent and have good training.

Update I have been thinking a lot about your question. I think we were missing the point (largely) because the students weren't asking about writing but about writing in English. Here's what I've been thinking.

First I think you should reassure them that if they are doing math in their first language, that is normal. Being able to do math in a second language is the last or close to the last point of fluency.

What I might suggest is that you say they can write in their own language, but then translate into English if they prefer. I'd have them hand in both along with the Google translate version so that you know they went beyond that (which is the obvious first step). Your goal then, even though you won't exactly grade on it, is to help them get more fluent in English.I think you should stress to them how important this will be in the workforce. You can also encourage them to write directly in English and say that you'll grade it to the same standard as the translated submissions, not expecting perfection.