Nunito font doesn't have glyphs for Cyrillic (I'm assuming other scripts too), leaving Cyrillic text in Arial or other sans-serif fonts.
Log: not sent
Same here, the 啪 in my name will display with a different, much bolder font. Note that the diacritics (and therefore also 啪) are included in the font: https://fonts.google.com/specimen/Nunito
But it seems it was subset to exclude those.
Example boldness in Cyrillic font.

This is just personal opinion obviously, but I'd like to vote against using the nunito font at all. Even outside of the missing glyphs, which is obviously a problem, it just looks a bit cartoonish. One of the first impressions I got after opening up /experimental was that I really didn't like the new font (although otherwise I think it looks excellent).
Nunito

Helvetica

So Nunito has a proper cyrillic variant - we just need to hook it up. https://github.com/alexeiva/NunitoSans/releases/tag/v2.500
However, @michaelnew - your problem is something else; Nunito should not look cartoonish (which I agree it does there, due to the weighting and/or antialiasing being wrong). Precisely what desktop env are you using there, and which version of FF? Have you manually zoomed the window at all, or are you using some other window-rescaling behaviour?
For context, it should look like this:

...which hopefully looks much less cartoony.
Even if I zoom things to roughly the same size that you have there, it should still look okay:

@ara4n Ha, that looks way better. Thanks for the screenshots. Turns out this was definitely on my end.
Apparently I had Nunito installed locally, but only bold/light/regular. I'm not sure what package was responsible for that, but deleting them from /usr/share/fonts/TTF fixed it. It only showed up this way in Firefox (not Chrome) for whatever reason.
Sorry for the noise. Maybe there's some commonly installed package that installs those fonts (I'm on Arch Linux using i3 in case that has anything to do with it), so if anyone else happens to have the same issue hopefully this helps.
Now that the font weights are fixed it looks fantastic. Excited to see the redesign roll out.
Example of japanese.

Yeah, anything non-ASCII is broken, not just Cyrillic.
Was subset too much when downloading from google fonts:
fixed for cyrillic and vietnamese at least. there simply isn't a Japanese variant of Nunito, so someone will need to propose an alternative font to use there.
@Ai-rin how obnoxious is the one shown in your screenshot? Or should we fall back to Open Sans (the previous font) for non-Latin/Cyrillic/Vietnamese?
Noto Sans JP is probably a decent alternative. Or you could drop custom fonts and just use system-ui defaults in the design?
@ara4n Honestly, the previous font was much easier for my eyes and I'd prefer to continue using it for everything (fonts setting in preferences?). So, i vote for Open Sans for now.
Greek is broken too.
It is somehow partially supported.
By partially I mean three letters (Delta, Sigma, Omega and only in capitals).
This leads to funny looking text like 
With Open Sans it looked more like this 
That said, I vote for Open Sans too.
Noto Sans works even better: clean look and less eye strain.
I'm using it with riot.im now.
In latest KDE Neon user edition, Nunito not-installed locally, greek letters 渭 and 蟺 are smaller than the other letters. e.g. :

I recommend using Fira Sans (or Fira Go, it's fork with support for more languages instead of falling back to another font).
Very nice font, works great!
Or, at least allow the user to change their font.
I used to like Nunito but fira sans is just.. better
We replaced the font with Inter.
Most helpful comment
Greek is broken too.
It is somehow partially supported.
By partially I mean three letters (Delta, Sigma, Omega and only in capitals).
This leads to funny looking text like

With Open Sans it looked more like this
That said, I vote for Open Sans too.