While I was busy trying out stuff for this question, I accidently removed the xcdatamodel-package.
I found out that recently there was this question which mentioned the following:
For some reason, the xdatamodel was removed from the build settings. I added it back in and it worked.
I tried doing what he did, but I don't know where to start.
I recreated the package, but when I try to set an attribute value, I get +entityForName: could not locate an entity named 'EN'...
All I know is; it did work until I started fooling around with not-tutorial-attributes and stuff.