There is no possibility to rebuild automatically the xpi with your locale throughout the WTS, up to now. So if you want to test your locale you can do this :

1. Download the extension (link in the WTS or in the forum topic)

2. Download your locale folder (zipped in tar.gz) from your language page (link at the top)

3. Unzip* the xpi and add one locale line in the install.rdf and/or chrome.manifest file.
[*Appropriate applications] Win/Linux: 7zip, Mac: Stuffit Expander, ***:***
Add one locale line in the install.rdf and/or chrome.manifest file.
You have just to copy-paste the en-US line and modify the letters according to your own language

4. Insert your xx-XX locale folder in the "general" locale folder

5. Re-zip everything in proper order

... and now you can test (on a clean new profile, it is safer)

If one of these steps is difficult for you, just ask on Help section