Navigation menu seeded with a non-en_US language can't be localised

http://forum.civicrm.org/?topic=16282 - if the database was seeded with a non-en_US locale, the civicrm_navigation contains non-English labels. This makes the menu non-localisable (to other languages); menu is localised upon rebuilds in CRM_Core_BAO_Navigation::getMenuName(), but that function requires that civicrm_navigation's labels are in English (otherwise there's no way to localise them).

We should fix this by either always seeding civicrm_navigation with en_US locale (we need to figure out how to pull the menu strings out of the template into POT files without localising the template...) or by storing the original English labels somewhere (and localising civicrm_navigation labels on language changes).

Another thing to consider is how to un-localise civicrm_navigation on upgrades (if the former solution above is chosen).