Is it permissible to recite the Holy Qur’an in the manner of some non-Arab countries, which alters the pronunciation of letters, such as pronouncing (tha) as (sa) and (dhal) as (za)?

The Qur'an must be recited with its correct Arabic letters as revealed by Allah. It is not permissible to substitute one letter for another if one is capable of pronouncing it correctly; otherwise, it would be a clear mistake (lahn jali) for which the reciter would be sinful. However, those who are unable to pronounce certain letters correctly, such as people from countries whose languages do not contain specific Arabic letters, are excused after making an effort to learn. They should not be put forward to lead prayers except for others like them. Allah has made the memorization and recitation of the Qur'an easy, therefore, one must strive to learn correct pronunciation and tajwid.