How does localization work?
Unless a publisher specifies otherwise, the ui_language parameter will automatically be assigned according to the locale of a user’s browser, e.g. the Spanish version of Firefox. If that language is supported by AddThis, the user’s menu should localize automatically. As a publisher, you can also override the language of your AddThis menu. For more information, please consult the language support and customization help page, which includes localized examples of buttons and button code.