Thank you all so much for your help!
Really? That's... really odd. Does it check the DS' language, or the language of the game cart it was caught on? Because I'm really, really 100% certain that the game I caught it on is Japanese. I'll try it again without using International. If that doesn't work, I'll try to find a place where I can find level 50 Pokemon.This indicates that you are not using international parents and should not be generating for them.
Or you have the wrong seed. but if you are finding seeds with delays in the 600's they are probably correct.
One other thing I noticed is that you are using rare candies. If you can catch level 50ish pokemon you could always use the by stat seed finder to save those.