I am looking at applying to a variety of Math Ph.D. programs in the US, and am starting to put all my ducks in a line so to speak. I know I will have two letters of recommendation from professors here at my university; one an influential number theorist who has guided me for two years on two separate research projects (in number theory), and one from a professor whom I have had for both undergraduate and graduate abstract algebra). My question lies with the third letter writer. I did a summer REU in mathematics, learned a lot, discovered a few (insignificant) partial results and developed a very close relationship with my research mentor there. He is from Mexico, got his Ph.D. in the U.S., and then returned to teach in Mexico. When talking about mathematics, his English is perfectly understandable, and he rarely has difficulties. However, when making plain conversation, or talking on subjects not related to math, sometimes he stumbles. I am fluent in Spanish, and read a portion of a text he wrote (in Spanish) on group theory-it was extremely eloquently written, read more like literature than mathematics. My question boils down to this: I would very much like for my third letter to come from my REU mentor, would it be appropriate to have him write the recommendation in Spanish?? I haven't a clue on this issue, please share all (if any) insight you may have.