Which is the best language to be taught and spoken?

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.

No doubt, the Arabic language is the best and most important language since it is the language of Quran. Learning this language and mastering it leads to a thorough understanding of the Quran and gaining vast Islamic knowledge. So, it is desirable that all Muslims, Arabs or non-Arab, learn the Arabic language. It is compulsory for every Muslim to learn enough Arabic in order to make his prayers correctly, such as reciting Suratul Fatiha.

This requirement does not mean giving up other languages. But a Muslim should try to learn as many languages as he can so he can convey the message of Islam to others besides using these languages in his daily life.

Allah knows best.