@CoralieCreupelandt Could detail the steps you followed to save new monitors directly from the monitor center? Did you try to call the newly created monitor from a script (this is actually where the bug happens most of the time). Did you use the PY2 or PY3 version?
I have tested the new Psychopy (PY2) version on 3 computers (with a french locale and windows 10), and also asked my french colleagues to conduct this test on their computers. None of us could use the monitor center (there was either a bug when saving the monitor from the monitor center, or a bug when calling the newly created monitor in a script).
Regarding your character problem, try adding u right before a string object (exemple: u"problème")