Bug 24661: Fix inclusion of locale-related javascript files

Bug 24661: Fix inclusion of locale-related javascript files

2 js files (Gettext.js, i18n.js) + 1 generated js file (locale_data.js)
were added by bug 21156. Their URL are broken and the files are not loaded.

Use Template::Toolkit plugin Asset to include these files, and exclude
locale_data.js when language is english, because obviously translations
are not needed in this case.

Test plan:
1. Just make sure the files mentioned above are loaded correctly when in
   English and in other languages as well

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>