Reading Quran with an accent in which some letters such as tha’ and dhal are altered
Question

Is it permissible to read Quran in the manner of some non-Arab people who pronounce letters such as tha’ as if it is seen, and dhal as if it is za’ and so on?.
Praise be to Allah.It is obligatory to read Quran with the letters with which Allah has revealed it, with sound Arabic pronunciation, as Allah says (interpretation of the meaning): 
“And truly, this (the Quran) is a revelation from the Lord of the ‘Aalameen(mankind, jinn and all that exists),
193. Which the trustworthy Rooh [Jibreel (Gabriel)] has brought down
194. Upon your heart (O Muhammad) that you may be (one) of the warners,
195. In the plain Arabic language” [al-Shu’ara 26:192-195] 
“Verily, We have sent it down as an Arabic Quran in order that you may understand” [Yoosuf 12:2]
It is not permissible for anyone to deliberately change any letter of it when he is able to pronounce it correctly. This is a kind of mistake which is a sin. 
If a person finds it difficult to pronounce the letters correctly – such as people in whose language some of the Arabic letters, such as tha’, dhal and kha’ do not exist – they have to try to learn the correct pronunciation, but if they are unable to master it then they are excused, but their example should not be followed, and they should be called upon to strive their hardest to learn and correct their pronunciation. And none of them should lead the prayer, unless he is leading others like him who cannot pronounce well either. 
Ibn al-Jazari (may Allah have mercy on him) said: Whoever is able to read the words of Allah with correct Arabic pronunciation but he deliberately pronounces it incorrectly like a non-Arab, out of arrogance, stubbornness and complacency, or because he is too proud to go to a scholar who could help him to correct his pronunciation, is undoubtedly falling short and sinning and being dishonest. The Messenger of Allah (peace and blessings of Allah be upon him) said: “Religion is sincerity: to Allah, to His Book, to His Messenger, and to the leaders of the Muslims and their common folk.” 
But if a person’s tongue will not obey him or he cannot find someone to teach him the correct pronunciation, then Allah does not burden any soul beyond its scope. 
End quote from al-Nashr fi’l-Qiraa’aat al-‘Ashr, 1/299 
It should be noted that Allah has made it easy to memorize and recite Quran, as He says (interpretation of the meaning): 
“And We have indeed made the Quran easy to understand and remember; then is there any one who will remember (or receive admonition)?” [al-Qamar 54:17]
Hence we see many of our brothers in the countries referred to who are able to pronounce the letters perfectly, better than many of their Arab brothers. So you should strive to learn the correct pronunciation and recite Quran with proper Tajweed, and not despair or give up.
 And Allah is the Source of strength.