This is due to the fact that MantisBT internally uses the PHP date() function [1], which does not support locales. As a consequence, all text-based elements (e.g. months, weekdays) are displayed in English (or, I suppose, whatever language PHP was compiled in).

As a workaround, I suggest you revert your date formats in config_inc.php to their default values. For example, $g_normal_date_format = 'Y-m-d H:i'; note the lowercase 'm', so month is displayed as a number.

[1] http://www.php.net/manual/en/function.date.php