'Can / can't' shows your present physical ability, whereas 'could / couldn't' is a hypothetical case when used like this. You can't speak Arabic because you don't know the language. If you know the language, you couldn't speak Arabic to those who don't know the language because they wouldn't understand you. Learning a language can be a real challenge. It's a fact. No one doubts it. Learning a language could be a real challenge if you are not prepared well enough or if you didn't learn the lesson. I hope that clears up the confusion!

EX 1 I would say "I can't speak Arabic" because of Ben's reason. EX 2 I would say "learning a language can be a real challenge" because it is something I have had experience in doing and am still doing now. Also like Ben said, its a fact. I would say "learning a language could be a real challenge" if I was preparing to learn a language in the future or if I thought I might not be prepared. :-)