i18n_menu_menu_link_update() makes menu_rebuild() 17% slower.

i18n_menu_menu_link_update() currently makes the menu_rebuild() operation (that is quite an expensive operation itself) about 17% slower.

So a 30s menu_rebuild() turns into a 35s menu_rebuild().

+ It adds 2 to 3 queries for each menu item, for a 1,250 item rebuild, that is 2,500 queries.

There's at least 2 ways to improve this method:

1) Only add to the $fields array the language field if it is us changing it's value.

2) Compare the new menu_link to the previous one to only trigger i18n_string_object_update() or i18n_string_object_remove() if relevant changes have been made to the menu_item.