Add translations for update system
Improve update API

Update script must implement 4 functions:
- apply_update() to perform the update (most important). Return true if
  all is ok, else false.
- need_info_update() returns true if we need more info for update, else
  false. If this function always returns false, you don't need to
  implement following functions (but it's better to not forget)
- ask_info_update() should be a HTML form to ask infos. Method must be
  post and action must point to _url('update', 'apply') (or leave it
  blank)
- save_info_update() is called for POST requests (to save form from
  ask_info_update())