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.
