It shouldn't be impossible - the format should be the same as Chromium uses and that is open source, so there must be a tool out there to edit it. But Opera's file should contain texts that are not in basic Chromium, and therefore you'd need to compare to an existing language to decide what a particular string means and so on. They might even be interested in using your translation, if the language is common enough. You didn't say what language it was ...
