Weird font in system windows

Suddenly all the system windows have this weird font. I can't read nor understand what the system is asking me. How can I change it to normal fonts?

I'm using OS X Yosemite 10.10.3


Solution 1:

Open Font Book and choose File → Restore Standard Fonts…

If that doesn't work, restart in safe mode (hold the shift when you hear the boot chime).