Update translations using our revised import-translations.sh script.

Fix locale-related issues.
  Renamed en locale to en-US (previously done during packaging).
  Added moz.build for integration with the Firefox build process.
  Enhanced the import-translations.sh script to omit incomplete locales
    and to generate jar.mn and chrome.manifest.
  Removed checking for incomplete locales from the pkg-prepare make target.
  Improved the output generated by import-translations.sh and pkg-prepare.

To avoid potential conflicts with other preference files, rename
our default preferences file from pref.js to torlauncher-prefs.js.

Do not load default preferences if the browser has already done so.