[3.2][WIP][ticket/12487] Remove legacy code

https://tracker.phpbb.com/browse/PHPBB3-12487

PHPBB3-12487
That's why I asked in the ticket. Maybe I wasn't clear about my intention... 

But by using the `lang` method we can use the implicite `sprintf` call, we are are that the `lang` array isn't modified, we don't have warning if the key does not exists and the overall code is more consistent. So I should remove types changes ? All of them or just those without `sprintf` ?

What about the removal of the `lang` call ? Like when calling `confirm_box` ?