Change core language files, Yosemite

Hello,

I think Yosemite was installed by choosing German as the installation language. After installation I changed the UI language to Hungarian.

Now I am having the problem that some of the apps automatically use German for notifications. I suppose some core language files "tell" them that the OS language is German.

Is there a way to change these files and replace them with the Hungarian ones? If, then which ones?

Thank you!