Is it compulsory for Muslims who do not speak Arabic to learn Arabic language?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Whoever can learn Arabic language is recommended to do so because through this language, one may recite the Words of Almighty Allah. Allah has revealed the Quran in pure Arabic language so one gets a plentiful reward. In a sound Hadeeth, the Prophet said: “If anyone recites a letter from Allah's Book, he will be credited with a good deed, and a good deed gets a tenfold reward. I do not say that Alif-Lam-Meem are one letter, but alif is a letter, lam is a letter and meem is a letter." Moreover, through this language, one can be acquainted with the rules of Islamic law and can read the Prophetic Hadeeth and the books of known Muslim scholars. Therefore, one can gain the good knowledge of Islam. The Prophet said: “If Allah wants to do good for somebody, He makes him comprehend the Religion (i.e. Islam).” [Al-Bukhari and Muslim]

If a person is incapable to learn the Arabic language, at least it is obligatory on him to learn as much as he needs in his prayers, like saying the Takbeer (Allahu Akbar), reciting Soorah al-Faatihah and other necessary matters.

Allah knows best.