How to fix screwed font display in password entry dialog?
So apparently it was solved by removing the fontd databases and deleting the files of conflicting fonts. (I guess the conflicting fonts were shown in Font Book?)

sudo atsutil databases -remove; atsutil server -shutdown


The password dialogs and force quit window are shown by processes owned by the root, so -removeUser might not work. launchd starts a new fontd process automatically, so atsutil server -ping is not needed.