Yes, you should have the right to set your language. In settings (Alt. P) in the Opera browser, make sure your user Interface is English, if that's what you want.
On the Google Search language, it's apparently basing it on country as set forth in your profile. Just a thought, though, since some countries have multiple languages. You know, if you go into your Google Account, you can select language, and language varient (https://www.google.com/settings/language).
I know Sgunhouse said: "Only way we have found is to edit the Country in the file local_state in your profile."
I was thinking though that maybe adjusting your Google account on the internet would help, and setting the actual language there.. First you go to, https://www.google.com/settings/personalinfo, (be sure to login), and then click on language. And set the language you want. If you're in a country with multiple language, that selection ought to do something 🙂 Anyway, it may be worth a try.