English text is getting displayed as squares.
On one of our users boxes, we are getting this weird behavior. He is on Windows 7 64 bit + decent Video Card. And he started to have that problem quite recently.
I have checked Language settings on the box and it seems to be fine
See the screenshot below, never seen this before. It is not only our WPF forms, it is any WPF form (below is simple window with Label/Text and Button in English).

Make sure the font you are using is installed in Windows/Fonts.