In this case, it would be best to politely ask the writer if they would feel comfortable having their letter reviewed by a native English speaker for grammar and clarity. This will ensure that the letter accurately reflects your qualifications and strengths.
It is generally preferred that letters of recommendation be written in English, as this is the primary language used in US schools. However, if the writer is more comfortable writing in their native language, it is acceptable to have the letter translated by a certified translator and submitted along with the original letter.
It is not necessary to disclose this information, as long as the letter is well-written and reflects your qualifications and strengths. The admissions committee will not discriminate against a letter based on the writer's native language.
It is always better to have a letter of recommendation from someone who knows you well, regardless of their native language. Admissions committees are looking for strong and personalized letters that speak to your character and abilities, rather than the language proficiency of the writer.
No, a letter of recommendation from a non-native English speaker will not hurt your chances of admission as long as it accurately reflects your qualifications and strengths. Admissions committees understand that not all letter writers will be native English speakers and will take this into consideration when reviewing your application.