Thank you for your notification.

I installed Opera Developer 126.0.5748.0 (DEB) and tested it.

In this version, the UI language has been replaced with the specified language.

I think this is a very good update and hope it will be implemented in Opera Stable.

Sorry for the late reply.

I have already uninstalled the snap version and am using the apt(deb) version.

I think your information will be useful when I switch to the snap version in the future.

Thank you!

cp /var/lib/snapd/desktop/applications/opera_opera.desktop ~/.local/share/applications/

many thanks!

I edited the file with an editor and was able to change the language.

/var/lib/snapd/desktop/applications/opera_opera.desktop

OK!!! it works

Bye

• I'm running Ubuntu 18.04.4 LTS
• Opera rel. 114.0.5282.21 (snap version)
  I added --lang=it (my preferred language) to the command line used by opera launcher
  
  Great: it works fine!!!

Now I will test Ubuntu 24.04.4 over my laptop and over VMVirtualMachine, but I'm optimist ...

• close opera

• open terminal

cp /usr/share/applications/opera.desktop ~/.local/share/applications/

nano ~/.local/share/applications/opera.desktop

• change the line:
  Exec=opera %U
  to:
  Exec=opera --lang=[language_code] %U
  where: [language_code] is example: pl

• save and close the file

• open opera

• enjoy

it's much easier as discribed.
In this case Linux Mint Cinnamon 21.3
Opera Flatpak Version

Got to your Startmenu all your way to the entry of the opera browser
right-click it an go on properties and add "-lang=Your Language ID"
as you can see in the screenshot below and your are done without messing with folders

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!

Acabei de revisar as configurações de região e idioma e estão todas em português do Brasil.

[I just reviewed the region and language settings and they are all in Brazilian Portuguese.]

[For example, access the settings and set Portuguese as the preferred language.]

[My Operating System is in Portuguese natively.

Below are the versions I'm currently using of the OS and Opera (with all updates available to date):]