How to change browser locale without affecting UI language?

Hi, I'm trying to change my browser locale without changing the UI of OS X.

In detail, I live in Austria - so it's DE-AT - but I changed the OS X UI to British English, so my boyfriend (who is Irish) can use it, too. But now, all multilocale websites redirect me to the British version. Not that it's a bad thing but a little annoying if I'm looking for a certain information localised for Austria.

I'm currently using Opera (but Safari works the same way) which links its locale to the system language preferences. If I switch that order, as I have done, the whole system (Finder, ...) changes, too. Is there a way, to avoid that? So I can set my browser locale to German (DE-AT) and leave the system (or perhaps just Finder, if not possible otherwise) in English? Also, iTunes sometimes mistakenly thinks, that I want the British store. I mean, it's actually my own fault since I set the preference like that but I just wanted to change the "display" language.

The Region setting in System Preferences is still set to Austria but that only affects the Date and currency formats. I'm not afraid of Terminal commands or other built-in tweaks as OS X has got a lot which aren't marketed by Apple.

Thanks in advance, Yi.