In filter view mode, use date filter field : months are always displayed in long format text. Displayed text is always english (no translation done by mantis).

Even if the localization is not possible for date when using config "$g_*date_format" values (see 0014589:0032553), the filter api force the use of "F" parameter for date() when displaying months values and that can be used to localize the displayed string.

In filter_api.php, row 3017:
echo date( 'F', $t_time );

Results can be localized when dislaying current date filters values :
echo langget( 'month' . strtolower ( date( 'F', $t_time ) ) );