Why? If you were designing a website with Mongolian as a language option you would choose a font that has support for Mongolian, not something completely unsuitable like Helvetica. The same goes for any language not written with a Latin alphabet.
If you don't want to provide a font and just want whatever passes for sans-serif on a computer with Mongolian fonts installed, use font-family: sans-serif; in your CSS and let the OS handle it.