I18n, better implementation

Missing the tests part, i'm really not good at that.

* 2 functions in classes/lib.php : 

   \_\_() // for translations, 
    function gatherTranslatableStrings($locale=null) // for finding '\_\_()' calls sources

* 1 route in Backend
* almost all messages adapted to call __() in php and twig templates.

(And sorry for closing the previous pull request, i'm new to this stuff, won't do that again.)