Month/weekday names can be set on all charts using the following properties of AmCharts class: monthNames, shortMonthNames, dayNames and shortDayNames.
So, to translate month names to German, you should set:
AmCharts.monthNames = [ 'Januar', 'Februar', 'März', 'April', 'Mai', 'Juni', 'Juli', 'August', 'September', 'Oktober', 'November', 'Dezember'];