I use the tip of matthiasmex. This is problem still stand in my current silverstripe-insatllation. my _config.php looks like this.
Its a strange thing that this umlautproblem only occurs on the monthselection (header above the calandartable) in the LiveCalendarWidget.
The translation of March is shown as "M�rz". The optionset below the table is working fine. No misspelled März.
I got no clue, where to track this error or to change the code effecting this.
Thanks for your help. Thomas
[Edit] It seems it effects only the $CalendarWidget and not the $LiveCalendarWidget.