It would be even funnier with the headcanon that Steve has an Italian family and speaks Italian.
He is confused to why she said it to him. Then he is confused why she speaks italian. Also he ask himself if she knows he speaks italian as well, maybe that this is the reason why she speaks it around him.
Robin will be mortified, because she did not know that he speaks Italian. Steve in his sometimes nonchalant not thinking too much ways, never gave away that he understood her and just someday either speaks to her in Italian or she hears him speaking to his family. Maybe she finds Italian books (like a cookbook) and starts to question a lot of interactions they both had.
In the end she loves it, because they can speak ill of customers or maybe a little bit of the party right in front of these persons. There can also be a moment where she is like "Of course, my Dingus speaks Italian he is my soulmate, my other half. The one person that understands me the best." (Maybe she starts thinking to teach him more languages) That would also be a really good answer from her when somebody asks why Steve can speak Italian (like the Party has never heard him speak it before). She is just like "He is my Soulmate of course he speaks Italian" If you want more you can also imagine Dustin taking the answer literally and fears that the upside down gave them superpowers (that this would also explain why they are so weirdly codependent in his eyes)
Okay something that I think should really be added to Robin is her knowledge of other languages.
You’re telling me this snarky ass ice cream server hasn’t cussed out a customer in their face in Italian?
She keeps saying “Mortacci tua” to Steve each time it comes up that Steve has lost several fights. He just stares at her funny when she doesn’t elaborate.
(The phrase implies that Steve has weak ancestors)
Robin stubs her toe “ZUT!”
(An exclamation like Damn!)
A attractive girl walks out of Family Video and once the door shuts Robin calls out “Fica!” Steve again just looks at her funny.
(Means Cunt but is used like a crude way of calling a woman hot)
When Steve is again harassed into driving around town, Robin calls him a “chupamedias”.
(Suck up/ bootlicker)
Thank you for coming to my quadrilingual Robin Ted Talk.
No, I don’t count pig Latin.