A primary language is that which is dominant at the time. Example: at home you might speak Russian. Russian might be your First language - the one you learned as a baby. At work, at an international company with lots of expats, the primary language might be English, as it is the lingua franca used between coworkers. the terms L1 and L2 are used to define first and second languages. There are no Third or L3 languages. You can have more than one L1 if you are raised bilingual for example. Many people have more than one L2 - My L2s are German and Russian, fro example.

Mother Tonge OR First Language - is the language that you learn, speak, hear and practice since you are born. In your case, your Mother Tongue OR First Language is Mongolian. Primary Language - is the language that you use on a day-to-day basis. It can include your mother tongue and other languages you speak. Second Languages - are the other languages that you speak, other than your Mother Tongue / First Language.