[ticket/12048] Fix the regexp for language keys in custom bbcodes.

PHPBB3-12048
Merge remote-tracking branch 'prototech/ticket/12048' into develop-olympus

* prototech/ticket/12048:
  [ticket/12048] Fix the regexp for language keys in custom bbcodes.
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/12048] Fix the regexp for language keys in custom bbcodes.