There is a masjid whose Imaam is a Haafidh of the Qur’aan and he has good understanding in the issues of the prayer, except that there are some defects on his tongue (i.e. in his pronunciation); but he is from a non-Arab country. Is his Imaamship valid alongside the fact that there are those with a clearer tongue than him?
I say: (If) he is not able to pronounce the (letters and sentences) of the clear Arabic language, then he may (read) some of the verses with other than its intended (meaning), such as substituting(ر) with a ( غ ) or a (ح) with a (ه )- saying: (الرهمن), which is similar to what some non-Arabs do; saying: الهمد لله and substituting the (ح) with a (ه); [(saying) الهمد لله رب العالمين instead of الحمد لله رب العالمين] or substituting (ر ) with a ( غ) and what is similar to it; then it is not permissible to pray behind such a person, rather he should learn because the meaning of (what he recites) is the opposite.
And if his defects do not change the meaning, then there is no harm in praying behind him. (If) his defects changes the meaning, it is obligated on him to rectify his tongue (i.e. his pronunciation) by learning the Arabic language. It is obligatory to get other people to be Imaam if he changes the meaning (in his recitation).
[Al-Fataawaa Wad-Duroos Fil Masjidil Haraam of Shaikh Abdullaah Bin Humaid (rahimahullaah) Muhammad page:315-316]