I'm using OpenXcom Extended 6.5.5 (https://openxcom.org/forum/index.php/board,22.0.html) but I think the method to change languages is the same as the version you are using:
(1) On the main menu click Options.
(https://openxcom.org/forum/index.php?action=dlattach;topic=9737.0;attach=54200)
(2) In the options screen click Display Language.
(https://openxcom.org/forum/index.php?action=dlattach;topic=9737.0;attach=54202)
(3) Select desired language from the Display Language menu.
(https://openxcom.org/forum/index.php?action=dlattach;topic=9737.0;attach=54204)