The original name of Buddhist is pali. But today, only sangha or
the monk who preached Buddhism usually study the scripts of pali
also known as Sanskrit.
There is no set language for Buddhism. Buddhists come from all nations and cultures and speak, read and write in the national languages of those locations.
Buddhists can speak any language; a dialect of Chinese, English, Portuguese, Czech, Tagalog, Klingon, or whatever else. Same goes for Christians, Muslims, Wiccans, and the followers of all different religions of the world; they're religions, not countries; language and spirituality are regarded as being two different things. You aren't born speaking Hebrew just because you're born a Christian, and you don't automatically learn Hebrew if you convert to Christianity. So same applies to Buddhism. Even though Gautama Buddha was born and lived in northern India, a Buddhist's mother tongue is determined by his or her country, parents, and/or interests...not necessarily the Buddhist religion.
Buddhism is not a language group any more than Christian is language group. Buddhists speak the language of the country in which they live (Chinese, Japanese, Korean, Vietnamese, Burmese, Thai, English, the languages of India, French, German etc.)
The Buddhist scriptures are typically seen in two languages - Pali and Sanskrit
The Buddha himself probably spoke a language that no longer
exists called "Magadhi"
They don't have a 'main' language they speak many different ones.
Buddhists live in almost every country in the world, and they speak the language of their countries. China, Korea, Japan, Laos, Thailand and India have large Buddhist populations, and they speak more than 1000 languages, including:
Buddhists speak many languages. That question is similar to asking what languages do American's speak. Your going to get a large and broad list. Research into the languages of South East Asia will help, but even so there are so many tribes w/ languages that are extremely rare and unknown to the overwhelming majority of people outside of the third world. I'm afraid that your not going to get the whole list without a serious amount of time given to research. Norman Chomsky may have some writings on similar topics.