Proficiency (or lack of it) of your recommender in written English has little impact on how adcom evaluates your recommendation. After all, they are not judging your recommender’s communication skills.
As far as recommendations are concerned, adcoms primarily focus on content.
(Note: Admission policies of schools and guidelines for standardized tests can change. Refer to their website for the most updated information.)
Probably, the best insights in this regard come from Stanford GSB. In response to the following two questions, they say:
Question: Our applicants come from all countries of the world and so some recommenders may not speak English fluently, if at all. Are language skills a barrier to writing an effective recommendation?
Answer: Let’s suppose that this candidate is coming from Brazil or from China and their supervisors’ first language is Portuguese or Mandarin. When we read that application, we will certainly take that fact into consideration. It’s really the content of the recommendation— not the spelling, not the grammar, not even the word choice—that’s going to be important to us as we read that recommendation. However, it’s always an option for the recommender—if that would make you feel more comfortable—to have your recommendation translated and to attach that translation.
Question: So in other words, when we read a recommendation from a non-native English speaker we still get enough information?
Answer: Absolutely. In my experience, the examples used in the recommendation and the content of what they’re telling us comes through. We are actively looking for individuals all over the world and that means that the direct supervisors, in many cases, will not speak English as a first language. So it’s something that happens often and we are very comfortable with looking for content and not how well it is written in English.
Most schools, though, are silent on this issue. And those who’ve issued instructions, limit them mostly to get the recommendation translated (not by you, but by the recommender) to English by a third party, if the recommender is not comfortable writing in English.
However, it’s unlikely that the adcoms, in general, treat the proficiency of your recommender (in written English) any different from what Stanford does: they too have applicants from several countries where English is not the first language, and hence they too face similar challenge as far as language in recommendations is concerned.
But, as is the case with many facets of MBA admissions, you should check your target school’s website (usually Admissions page) and/ or contact them to know their take on the issue.
Conclusion
What your recommenders say is much more important than how they say, and hence their proficiency in written English is unlikely to have an impact on the evaluation of your recommendation.
In case, the language is bad (just too many errors), it’s better to get it translated, which most schools recommend.