White blocks in place of the numbers on the lock screen
This happens on many Samsung Galaxy devices (S3, S4, S5, Note 2 and Note 3). Android forums are full of this and it has nothing to do with language settings.

Long-press power button to restart phone clears it up for some time (days to weeks), then it comes right back. As of this writing, there's no real solution, but apparently it's memory related. Did your phone perchance start giving you occasional random error messages "Unfortunately contacts has stopped working"?