Answer: Arabic language
Explanation: Islam as a religion originally emerged after the vision that Muhammad had and what he later wrote as the sacred book of Qur'an. Muhammad was Arab and Qur'an was originally written in Arabic. Like any other religion originally recorded in the area where it originated, and in its native language, Quran has its original Arabic version. The holy book Quran is translated into many languages, but there are always opinions and claims that this book can be truly understood in its original language in this case Arabic. However, there is also something that is related to the Arabic tradition and the characteristics that are rooted in the Arabs from which the Qur'an came from, so in order to best understand this holy book it is necessary to know the Arabic language and also to know the Arab tradition.
Similarly, for example, with the Old Testament, it can be understood in a best way if the Hebrew language and Hebrew tradition are known. Or also, some experts believe some of the Gospels can be truly understood if we know the Aramaic language which Christ spoke, and on which the Gospels were originally written.