Browser language

According to http://www.w3.org/International/questions/qa-lang-priorities this is simple:

Firefox
To change your language preferences go to Tools > Options (Windows) or Edit > Preferences
got to tab Content. Down right of Languages click on the button Choose.

* You can add languages to this from a (long) pre-defined set, and Firefox will send all the languages you choose with the Accept-Language header in the order in which you arrange them.
* You can't create custom language tags using the standard interface. (You can, however, via about:config, using intl.accept_languages if you are geeky enough.)

Safari
On Mac OS X the language sent is that specified in the OS X system language preferences. To change those preferences go to System Preferences > Language & text > Language. You can have a long list of languages in this dialog box, but Safari will only send the top item with the Accept-Language header. You can't create custom language tags.

On Windows, you can't change the language sent by Safari. The language sent with the Accept-Language header is derived from your system settings.

Google Chrome
To change your language preferences go to Options (Windows) or Chrome > Preferences (Mac OS X), and select Under the Bonnet > Web Content > Languages & Spellchecker settings.

You can add languages to this from a pre-defined set, and Chrome will send all the languages you choose with the Accept-Language header in the order in which you arrange them. You can't create custom language tags.

Opera
On Windows you can change your language preferences at Menu > Settings > Preferences > General > Language > Details (button).