Okay. You know that tweet Astruc made about Nino's hat, about the reason Nino was so sad his hat got hit by an akumatized supervillain that one time because it was supposedly (a gift from?) Nino's older brother, who is no longer with them? The tweet that may or may not be complete bullshit, because Astruc is well known for trolling the fanbase?
Right, background established.
So. Assuming that tidbit was legit, and Nino actually has an older brother who is gone:
Nino really, really looked up to his older brother. He thought his older brother was the coolest person in the world, and it's his goal to be at least as cool for Chris as someone to look up to. Nino's brother really emphasized that Nino should be true to himself, and not care about what other people think, or change how he acts for other people, because the
important people in his life won't care about Nino calling them "dude" all the time, or stuff like that.
In other words, Nino's older brother is the reason why Nino has so little experience with
code-switching.
Nino is having to learn how now, between Adrien's dad being a mega
abusive hardass and his own secret identity, but he hasn't had much practice with it, and his older brother's support of him is a major reason why.