Click on Manage my extensions in the Main Menu-> Web Translation System on the left side.
In the new page that will open you have to pick the extension you wish to complete with locales.
Click on Download all locales as tar.gz-file (you can also download one chosen locale only by selecting the desired one, then clicking on the download link)

Note on TAR.GZ files: when all locales are downloaded in a single tar.gz file, the missing strings will be replaced by their English (en-US) counterparts.

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

For more information, please see the other Questions & Answers