Hi
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
Bildschirmfoto vom 2024-05-26 13-57-46.png