BUG: Load translations before instantiating the GUI

Moved loadTranslations() and loadLanguage() methods from qSlicerApplicationHelper
to qSlicerCoreApplication. This enables loading them before instantiating the
main Slicer GUI.
Many thanks to zengwenye <iamzengwenye@163.com> for submitting the patch.
Fixes issue 2836

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21559 3bd1e089-480b-0410-8dfb-8563597acbee