@yokoopera - always work on a copy of the activator

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

On Ubuntu:

• 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