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.

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.

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.