I don't think the support is really there for vertical writing systems.
The mouse scroll scrolls right, selecting text works (with a horizontal cursor), the western text (dates, numbers and "Covid-19") embedded in the text are appropriately rotated.
"View source" shows the source text, and it doesn't look like there are any nasty tricks to support this.
Is anything missing?
Would you please share a link where I can download Mongolian Helvetica?
Having said that, the place to look for multiscript support in Helvetica would be Linotype’s Neue Helvetica World: https://www.linotype.com/5553118/neue-helvetica-world-family... . Unfortunately, while they currently support 103 languages, Mongolian isn’t among them. I guess if this restoration takes off it might become part of the next release.
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.
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.
This is hardly a coincedence that Cyrillic alphabet has broader support and adoption among fonts.
It's interesting to look at the Mongolian president's web page (https://president.mn/mng/): It scrolls horizontally. And see how it responds to the hamburger menu, for example.
Edit: Oh wait, the president's web page is itself a good example: Here is the English version (https://president.mn/en/).