Font changes during the runtime in WPF app

I'm using a non English (Hebrew) font i开发者_开发技巧n a WPF application. The font was embedded in the project by expression blend.

In the designer I see the font correctly, but during the run time part of the letters are being changed to different font style.

What could it be?

Are the design and runtime environment on the same machine ? If not, check for installed fonts. If it's on the same machine, then maybe check if some localization happens on during runtime.