In ticket http://bugs.mitk.org/show_bug.cgi?id=18955 this issue was already addressed but it only fixed the concurrent access for a single instance of DICOMITKSeriesGDCMReader. If you try to load multiple images using multiple instances of DICOMITKSeriesGDCMReader, then using 'setlocale' will still cause a data race and probably crash.
The easy fix would be to make the mutex protecting this a static variable.