Islam and Arabic Language

In 571, Mohammad the Prophet took his first order from the God, which is "read". Then after many orders, Kor'an the holy book was completed. After the completion of Kor'an, it has been single and certain guide for all the muslims. Yet, if a person want to read and understand that book, that person has to know Arabic Language. The problem is why is Arabic main language of Islam and Koran even though the islam is universal? Before the Islam, arabic was just one of the Language that was spoken in only certain regions in Arabia. But after the Islam whole Arabia had started to talk in arabic because arabia is the region which is starting point of Islam. It was natural but after spreading other parts of the World, The arabic had became main language of some regions which was unnatural. In christianity, for example, for many centuries, Latin was the language of Bible but it could not influence people that made people's who live in Europe lanugage change. Because unlikely to Islam, people can prey and worship in other language. These restrictions in Islam influence people's language because worshipping and preying take a lot time and they have to be done everyday. In addition to that Kor'an has to be read in Arabic. Due to these reasons from tunus to Iraq arabic is the main language even though it is not their ancient language.
Due to many reason arabic language has spread but some countries such as Iran and India has preserved their language altough they are muslims. The answer lies under their culture. Changing religion just not change belief of community, it also changes culture of community. Thus Iran and India which posses strong culture and language whose roots are older than 3000 years old. It is impossible to change cultures that old. After the Islam, India has become a Great Britain Empires Colony and Iran was ruled by Seljuks yet the results were same these cultures could preserve itself.
In a nutshell, spread of arabic language inevitable because of