In the French version of MT-4.2 (MT-4.2rc5-fr), when using a custom field of type Image, the media selector modal window breaks because of the translation of "View Image" into "Voir l'image". The single quote isn't escaped in the javascript code returned by MT, which stops the rendering, and the modal dialog stays as a blank window on top of the edit screen. If you're blocked by this bug, select English as your prefered language.
Reported on Movable Type 4.2
You've filed this in the bug tracker, yes?
Yes I did yesterday, I was just giving a workaround in case someone else gets stuck with this bug ;-).