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  1. Download the file (replace %LOCALE% with your locale string) and open it in a text editor of your choice
  2. Don't translate all strings in the file!! :) Search for the line including "### start of untranslated words" and start from there. Be sure to remove the // at the end of each line you're translating.
  3. Save and e-mail the file back to me (djst at mozilla dot com) and I'll upload it on the server so we can try it out.
Note: You can check syntax of the language.php file with php command:
php -l language.php