@leocg sorry for reposting, i thought their may be other way to request for a new language addition process here.

I do not use ubuntu or gnome (or unity, if the screenshot is from 16.04 or older).
All the opera's .desktop file mentions is 2 entries that are only available under unity: one for launching it as usual and one for launching it in private mode, so I assume the other 5 come from gnome or from some gnome extension.

However, since opera is closed source, there is no ppa for it, you won't find anything to translate it yourself.