Praise be to Allah.
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.
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