@the-codifier said in Can't change UI language. No option "Display opera in that language":

Re: Can't change UI language

I had the same issue when I recently installed Opera Stable on Fedora 36 KDE. My native language is spanish, so I will use this language on this tutorial. Probably it will work for all Linux distros.

1. Go to this route on your File Manager /usr/share/applications
2. In this route, you find many files with .desktop extension. These are the icons you use to open the programs. Find the Opera Browser one (usually called opera.desktop)
3. Right click on it and open with your favorite text editor (I use Kate in KDE)
4. The first section must look like these

[Desktop Entry]
Version=1.0
Name=Opera
GenericName=Web browser
Comment=Fast and secure web browser
TryExec=opera
Exec=opera %U
Terminal=false
Icon=opera

Locate the line Exec
5. Add the text lang=your language after the word opera. Example for spanish

Exec=opera --lang=es %U

6. Write the --lang flag in all places where you find Exec on the file
7. Save the changes on file. Probably the text editor claim you for the super user (root) password. Type it.
8. Double click on Opera icon. Your browser now opens on the language you selected.

The reason of this is: Opera is charged with all the languages, but the instruction that launches must be declared with the flag --lang to be opened on the language you want.

I hope I have been helpful.

Thanks, though the topic is old, in my case it was helpfull and it worked!