[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
[ticket/12527] Fix "board’s language" vs "boards language"

PHPBB3-12527
Merge pull request #2440 from nickvergessen/ticket/12527

[ticket/12527] Remove translation editor from ACP

* nickvergessen/ticket/12527:
  [ticket/12527] Fix "board’s language" vs "boards language"
  [ticket/12527] Use a proper language string on confirm screen
  [ticket/12527] Remove unnecessary language strings
  [ticket/12527] Check whether the language exists
  [ticket/12527] Restore missing files and variables check
  [ticket/12527] Remove editor from template
  [ticket/12527] Remove language file editor form ACP