I just found the problem. Chrome had bad data saved (and I guess has had it for years at this point.)
Just to share, in case it's helpful: After checking the system settings as you suggested and finding no Swedish, I decided to rule out the browser itself. I tried Firefox. It worked fine in English. Then to sanity check, I tried Chrome Incognito - also worked fine in English. I cleared cookies, but it didn't help.
Finally, I started digging into DevTools in Chrome. I found the place where VNC looks up the available languages. It's here: <server>/vnc/dist/main.bundle.js
So I stuck a "Watch" on that variable. Even tho my Chrome installation only showed 1 "Swedish" installed, and it was set to the Lowest Priority, the "Watch" showed a second one, near the top. I guess VNC doesn't have a distinct en-GB language, so it took "sv-SE"?
Anyway, long story short, completely removing Swedish from Chrome settings got rid of it, and now adding it again correctly shows it at the bottom of the list. And VNC works properly now.
Sorry for wasting your time, but thanks for the inspiration to track this down.
