Thank you for this tip. It was a great help. In my case I needed a Spanish translation.
My installation was done by snap and I simply edited the access shortcut like this:
env BAMF_DESKTOP_FILE_HINT=/var/lib/snapd/desktop/applications/opera_opera.desktop /snap/bin/opera --lang=es %U
Note: Sorry if the message is not well written. English is not my native language and I used the google translator.