To change the language your account is displayed in, go to your Dashboard (if you're in the editor click on the "back arrow" icon next to the Weebly icon to exit the editor).
Scroll all the way to the bottom of the page, and in the lower right corner is the language currently displayed.
Clicking on it will bring up a list of languages.
The language you select will change the language we display within your account pages and within the Editor. However, this does not translate any of your own text that you've created on your site.