[ticket/12527] Remove language file editor form ACP

PHPBB3-12527
[ticket/12527] Remove editor from template

PHPBB3-12527
[ticket/12527] Restore missing files and variables check

PHPBB3-12527
[ticket/12527] Check whether the language exists

PHPBB3-12527
[ticket/12527] Remove unnecessary language strings

PHPBB3-12527
[ticket/12527] Use a proper language string on confirm screen

PHPBB3-12527