How do admission committees in US grad schools avoid a 'Native Speaker Bias' in evaluating recommendation letters? For the first time (I am a relatively junior professor), a student has asked me to write a letter of recommendation for him for graduate school application in the US. I was glad to do so, as he is an excellent student who has performed research with me - we have authored a paper together.

On going through my letter now, it seems a bit ... unnatural. I have used the same adjective on multiple occasions - I only know so many ways to say that a student is 'good' at something. I am also sure that some of my word-choices will be noted as peculiar to the native speaker - most of my English writing experience is in academic contexts. While comparing to random examples that I have found on the internet, my letter seems boring and dull. I am also sure that my letter will have grammar errors of some form or another.

I wanted to know how admissions committees avoid having biases towards letters written by more talented writers, or by native speakers. If you were to put two letters having similar contents in front of me - one written by me and one written by a British or American or any other native  English speaking professor, I would say that theirs is definitely superior, and I would feel more positively to their student.
I think your question falsely equates being a "talented writer" with things like knowing correct English grammar and spelling and knowing many synonyms for "good", which native English speakers are better at than non-native ones. Those things may indeed be correlated, but they are far from the same, and there are many native English speakers who are crappy writers, and plenty of non-native ones who are "talented writers" who can write a very compelling letter of recommendation, whether grammatically correct or not.

To me, a talented writer is someone who has a good ability to communicate complex ideas effectively in writing. In the context of a letter of recommendation, the grammar and spelling may be poor and the choice of words can be clunky in certain places but the letter can still tell a good story about why the student is worth admitting and be more persuasive than a dry, boring but grammatically flawless letter written by a less talented writer. So, to the extent that students whose letters are written by talented writers enjoy an advantage in admissions decisions, that advantage is not what I would call an unfair advantage or a bias; at least, it's no more unfair than many other merit parameters that applications are judged upon, many of which are unfair to some extent. (In particular, with letters of recommendation there is certainly some unfairness that is intrinsic to the whole system of LORs that academia is built on, since how effective the letters are depends on many random factors, including how talented the writers happen to be).

To summarize, as a general rule I don't think there is any particular "native speaker bias" that needs to be avoided. Admissions committee should read the letters and be impressed by each letter precisely to the extent that it makes a compelling case for admission.*



*The only possible exception to this conclusion is in the case of letters coming from countries where many academics really can't speak or write English very well. Those letters in some cases can be extremely difficult to extract any useful information from, and an applicant with such letters is probably not going to fare too well, perhaps unfairly. However, I don't see what can be done about that -- that is essentially one of the flaws intrinsic to the entire LOR system, as I mentioned above. The system may be flawed, but as far as I know no one has yet found a system that works better.