What is the ruling on praying behind an Imam who pronounces some words of Al-Fatiha and the Qur’an in the colloquial dialect, such as one who pronounces the word “ghayr” (غير) with a kasra on the ghayn?

Breaking the "ghayn" in "ghayr" (غير) in Al-Fatihah is an obvious, disliked error (lahn jali) that does not invalidate the prayer or change the meaning. It is, however, disliked to pray behind someone who makes this error. Imam Al-Nawawi mentioned that errors in recitation are absolutely disliked. If the error does not change the meaning, it is a reprehensible dislike (karahat tanzih), and the prayer is valid, as is praying behind such a person. However, if it changes the meaning, the prayer of the one whose tongue allows him to make this error and has not learned is invalid, and it is not permissible to pray behind him.