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.
Sure, I can go with font-family: sans-serif
But what if I have to add Mongolian support for a website using Helvetica typeface for the currently used Cyrillic script?
You can't, but this is nothing new. Fonts tend to support a subset of scripts. You just got lucky with Cyrillic that a Helvetica version supporting it exists.
Here you would choose a Mongolian font and add it to the font-stack in CSS for that language, just as you would for Arabic or Chinese. If you want a grotesque typeface that matches Helvetica in style for Mongolian, use one.
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.