[ticket/12858] Use translated string instead hard coding

PHPBB3-12858
Merge pull request #2744 from carlo94it/ticket/12858

[ticket/12858] 'GMT' is hard coded and not pulled from language files

* carlo94it/ticket/12858:
  [ticket/12858] Use translated string instead hard coding
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12858] Use translated string instead hard coding