Is there a difference between the pronunciation between 'ca' (with the squiggly thing) and 'sa'?
No there is not. Sa and ca sound exactly the same!
P.S That squiggly thing is called cedilla (suh-dee-ah)!
I adore the fact that "His skirt is green" is (rightly) accepted. Just wished the sentence was "Sa jupe est rose"...
Should also accept Ça jupe est verte