Tried to post a solution here, but Opera Forums doesn't want to publish it. Only thing Opera cares is to place promotion into the Browser.
Posts made by Saragoo
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Opera for LinuxRE: Can't change UI language. No option "Display opera in that language"
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Opera for LinuxRE: Can't change UI language. No option "Display opera in that language"
OK, there is something that may work.
I opened my filemanager (Nemo or Thunar or ...) with the terminal (sudo nemo) and then i go to /usr/share/applications
Right-click on Opera and open the attributes menu (in German "Eigenschaften", i do not know if the translation is correct).
Change the command opera %U into opera --lang=de (de for german, choose your language)
Close this. Now Opera should open in the correct language. I hope it works.