This contract is full of weird grammar. Was it translated from another language?
If the payment deadlines are written in the contract, then by paying late, they have already broken the contract on their end multiple times. However, trying to get out of it using that may involve lawyers....
So... according to the excerpt, as best I understand it:
Party A (you) will keep your webcomic updated to the best of your ability within reason.
If you decide to cancel the contract, Party B (the site) has the right to request back any revenue share you've been given so far, AND on top of that, to get you to pay out for the money they've spent translating and promoting your work.
Honestly... I have no idea if this contract is viable or not. I've never seen anything quite like it, and definitely not signing somebody on for such a substantial period with no break clause. you'd definitely need to talk to a legal expert before just ducking out of it, because the amount they could claim to having spent on translation and promo could be significant, and paying back everything your comic has ever earned you.... yikes.
Hopefully, a legal expert would be able to get you out by arguing that they have not been keeping up their end of the bargain, which probably should include that Party B has responsibility to do their absolute best to pay Party A what is owed within a specific time period AND to make every reasonable effort to promote the work.