The imam of our masjid have problems pronouncing many of the arabic words while reciting the Qur'an. I was told that of the imam can't recite the Qur'an, pronounce the arabic words as good or better than you do you can't pray behind him lest your prayer is invalid. When I heard this I stopped praying behind this imam but have not said anything to him yet. Can you please calrify this matter for me.
If the imaam is unable to pronounce some words because he is a foreigner (non-Arab), for example, or because he has an accent, then it is OK for him to lead others like him in prayer. If there is no one who can pronounce the words properly, then his leading them in prayer is valid and the prayers offered behind him are valid. But if there is someone who can pronounce all the words properly, in that case it is not correct for someone like the imaam mentioned in the question to lead the prayer.