#Why_is_Quran_in_Arabic_language?
The Glorious Qur’an was revealed in a high classical Arabic. Due to the fact that the miracle of Qur'an is everlasting and because prophet Muhammad peace be upon him is the last prophet, then the book that has been revealed to him has to be everlasting too. 

Almighty Allah addresses with this Qur'an all nations through all ages; that is why Quran is revealed in the most expressive of all languages which is the "Arabic language", for many reasons to be highlighted.

📌First , It is impossible to make any changes in Qur'an since Arabic is a living language.

📌 Secondly, Quran was revealed to Arabs in the language they master. Similarly, previous Scriptures like Torah, Gospel , and Psalms were revealed in Hebrew; the language of the people of that land.

Allah says in the Glorious Qur'an:

“And We did not send any messenger except [speaking] in the language of his people...” [Qur'an 14:4]

📌 Thirdly, Arabic is very rich and effective despite being less in letters than other languages. In a short way, it can convey plenty of messages.A word can spontaneously bear more than one meaning,  and this does not exist ever but in Arabic.

📌The Arabic language has such more qualities of existence and durability that makes it the most lasting semitic language of all،  so it- by default- also suits to be the language of the Final Lasting Divine Book.

📌 Something which can't also be ignored is  the powerful influence of  the Arabic language on people that it changed the first language of vast area to become Arabic instead of other local languages as soon as Islam spread in their homelands , like North African Countries. A thing that has never happened with other scriptures that were instead translated to the languages of their followers.

#Reply_to_whoever_argues_that_Quran_was_revealed_in_Arabic_and_is_very_difficult_to_understand

First,  if this is the main issue of doubt, then you should also question the Gospel, that was initially written in Aramic which is  a language that is not understood by most humans at the very same time that Christians claim that Jesus was sent to all nations!!

Second, to your knowledge,  During the Andalusian Era, the Era of the scientific and cultural Prosperity, Non Arab students used to boast by learning and practicing Arabic,  all the science and knowledge that was transmitted to the western world on which their civilization was built was transmitted in Arabic. 

Lastly,  if the Arabic language *_as you claim_* was a boundary,  millions of people wouldn't have embraced Islam, and the rate of Non Arab Muslims wouldn't have been way higher than tha rate of Arab Muslims themselves,  and you can check that out from many international accredited sources.