For Muslim, Arabic Language is very crutial important that covers in most aspect of worship ritual. When conducting Prayer (Shalah/Salat), Arabic Language is used from opening (Takbir) until the end (Salam), The voice to call to prayer (Azan) is also by using Arabic Language and can't be changed by the another language. And of course what make Arabic Language is more special is this language will be used in the paradise at the day after to unite the entire paradise society.
Arabic Language is the second International language that used by most population of the world after English as Lingua Franca of the world. The language which mostly spoken by the people who stay at Arab land will develop day to day since the development of Islam religion.
Many Islamic teaching book (Kitab) are provided in Arabic so that the Muslim should at least understand Arabic passively. That why Muslim must learn Arabic for the religion need.