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.

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.