I saw someone post on here something along the lines of "main language next to nameplates" but it got me thinking about why not have the flag of the country their from?
When you're clicked on your menu and you hover over someones name it comes up with all the things that the avatar includes that might be blocked in your safety settings. Right next to those, something that you can only see from your menu (so it's less messy) should be the flag of where their from. I feel that would be an easier indication of their main language as opposed to "EN", "FR", etc for trying to figure out what the language acronym stands for.