First of all, you must register as official translator for the chosen extension

Click on the upload link at the top or at the bottom of the locale page.

Select the language you wish to upload. Use the browse button and find the locale files on your computer. Do the same for each file of your locale, including contents.rdf. Validate your selected uploads with the "Submit Query" button at the bottom of the dialog window.

Note:you have to be logged in order to access to the Web Translation System

For more information, please see the other Questions & Answers