It seems that different web sites utilize different parameters to detect your language. So you may want to try both of the following methods.
The first one is the User-Agent Locale parameter. To change it:
* input "about:config" into the Address box, and press Enter;
* Firefox wants you about the danger. Read and click the button to continue;
* input "useragent.locale" in the "Filter" box;
* double click the "general.useragent.locale" parameter and change it to your language, such as "fr-FR". (If you want to set it back, right click the parameter and choose "Reset" from the context menu.)
This changes the language part of the "User-Agent" in the HTTP request header to something like this:
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; fr-FR; rv:220.127.116.11) Gecko/20100202 Firefox/3.5.8 (.NET CLR 3.5.30729)\r\nAfter the change, go to www.wikipedia.org. It will pre-select your language for you.
The other one is Accept-Language parameter. To change it:
* open the "Options" dialog from the menu "Tools|Options" (or "Edit|Preferences" in Linux);
* select the "Content" icon;
* in the "Languages" section, click the "Choose..." button. A "Languages" dialog pops up;
* add the language you want from the drop-down menu "Select a language to add...";
* in the above language list, select the newly added language and use the "Move Up" button on the right side to move the new language to the top;
* click "OK" button to finish.
This changes the "Accept-Language" of the HTTP request header to something like this:
Accept-Language: fr-fr,en-us;q=0.7,en;q=0.3\r\nAfter the change, go to www.mozilla.com. You will see the Mozilla web site talks to you in your own language now.