How to approach student in lab with very bad English? Short version
I am a senior postdoc in a research lab mentoring a first-year graduate student from another country, and he has very poor English:    


Would it be rude or unethical or just silly to suggest he improve his English?    
If it is appropriate, what should I suggest he do, and how can I do that without coming off rude or racist?    


Longer version
Person X is bad enough with English that it is holding back our ability to do science. It is very hard to understand what he is saying, and he has trouble understanding very basic (e.g., first grade) level sentences spoken fairly slowly. It is so frustrating at this point that I basically try to communicate via email, which is sort of unfair to him. I am his main source of scientific guidance in the lab, as the PI is not usually around.

If it matters, he is from China and in his 20s (I only bring that up because maybe there are cultural or language issues here I should be sensitive to, but also he isn't a teenager so maybe he will have trouble improving).

I should add that most students, by this time in their tenure as grad students, have improved dramatically in their English (we have a lot of international students). His case is unusual in that he has really not improved at all. My guess is that this is because he speaks his native tongue at home (I have noticed this) and doesn't speak English much outside of lab.

Communication is crucial in science, and this person cannot communicate well (in English). OTOH, I worry about violating all sorts of professional or personal ethical boundaries, and offending him, if I bring the topic up.

Another option is that it is pretty much useless. That is, he is in his 20s, and isn't going to improve significantly, so maybe I should just look for advice on how to make the best of the situation. I'm not saying this is true, just throwing it out there as a possibility.

There has been a related question about students in classrooms. This is different, as I work with this person almost every day in a laboratory setting (
Should we tell students they need to improve their English?).
As an adviser or mentor it can be interesting to learn how the person you are advising/mentoring is feeling. Based on the response, you might gain a clearer idea on how to approach finding a solution. It seems that the individual in question understands written English quite well given you mention you have chosen to communicate via email. I would suggest sending them a message asking them how they are feeling with the program so far and with their work in the lab, and what are some challenges he has faced or is facing. You could ask whether he feels his English proficiency has been problematic so far, and add a couple of other items to like working with the equipment and colleagues, for example. This opens a space for conversation that is minimally face-threatening to the individual and invites them to seek advice and guidance to help their better functioning in the lab.