Update:
With this hack, Slicer starts but files are badly read (NRRD, DICOM…).
This fix yields a functional Slicer:
locale.setlocale(locale.LC_ALL, "C.UTF-8")
NOTE: this is just a temporary hack for my Linux build at the moment. It should not be needed at all and things should bet back to normal when the library/toolkit/infrastructure upgrade phase is over.