Language Switcher is an add-on that let you easily change your browser's language from toolbar popup.
Simply open the popup UI and select your desired language from the drop-down list. Please note that all changes are auto-saved to the memory. After changing the language, please reload the page to see the updated HTML. There is also a reload button in the popup in order to reload the active tab. Right next to the reload button, there is the reset button to revert languages changes to factory settings.
Note #1: this add-on changes the "accept-language" header that your browser sends to web-servers.
Note #2: in some websites, you might need to clear cache & cookie to see the language changes. Because they may use a cookie to store language preference and no matter what you choose for your browser, it won't change until you clear cookie & stored data.
Note #3: if you have a feature request or found a bug to report, please fill the bug report form in the add-on's homepage (http://mybrowseraddon.com/language-switcher.html).