Setting Language Preferences in Internet Explorer
In Internet Explorer, if you do not see the "Tools" menu at the top, click the "Alt" key on the keyboard to reveal the menus, then go to Tools > Internet Options and you will see a dialog box like this:
Click the Languages button near the bottom and the Language Preferences dialog box will appear.
Notice that I already have quite a few languages configured, the languages should normally be set so that the most preferred language is at the top and languages are listed in decending order of preference. If you need to add a language, click the Add... button and the Add Language dialog will appear as shown below.
Setting Language Preferences in Firefox
On Windows, Firefox has language settings under Tools > Options just like Internet Explorer. On Linux, Firefox has language preferences under Edit > Preferences.
Note that for Firefox, to add a language, you select it from the dropdown list, then the Add button becomes enabled. As with Internet Explorer, languages are in decending order of preference.