This bit stood out for me as well. Wish Khushi was not shown to be like this but its ok lots of people in the world are not comfy with english. But by that it doesn't mean one is not intelligent or educated. Just educated in a different language.
Anyhow i think it was to show that while khushi was trying to wish Arnav in his way, it didn't matter to him. Like he didn't think she was any less because she couldn't say it.
Something like they have accepted each others differences and are bridging that gap.
Not sure but thats how i felt.