Technically, Leliana loves everyone and she is more Ferelden at heart than Orlesian. So I doubt any disagreement between the two is based on that. I can see Leliana having issues with Cassandra's personality though.
Well, Leliana considers herself a Fereldan at heart, but no matter what she might think of herself, her demeanour, accent, fashion sense, connections, experience, job and outlook at life are heavily influenced by Orlais. But yeah, I think it will come all down to personality. Heck, in DA:O few seemed to care about Leliana there despite all the bad history between Ferelden and Orlais.
The Nevarra vs Orlais thing has more chance of playing out with Vivienne or another companion (if we get a even more Orlesian one).
Oh, yes, it seems Bioware wanted to outdone themselves by having the most Orlesian of the Orlesians in your party. Leliana is going to look like a barbarian near Vivienne
Of course, Leliana probably won't be a companion, so we need another foil for Cassandra. Since Vivienne is also a mage, that's even more contrast! Looking forward to their party banter.
I also wonder, with all the Nevarra tension with Orlais mentioned in lore, how Cassandra is received there. She is part of Nevarra ruling family and I doubt the Chevaliers are happy that a Nevarran (and a woman) was named Hero of Orlais.
Well, she saved the Divine in a rather spectacular fashion before thousands of Orlesians, so at least the common people shouldn't antagonize her. The nobles, on the other hand... (those important book-stabbing lessons might help her save face).