Associating people with languages

Just about every one of my friends and family is Hebrew-English bilingual, (some are also proficient in other languages as well) with some being stronger in or preferring one or the other.  The language I speak to each of them generally depends on which language they prefer or in what context I met them in, but I also am pretty casual when I speak in terms of letting words from the other language slide in, since I know I'll be understood.  For some people I know, I can go either way in terms of which language I speak to them in, but for others I so strongly associate them with one particular language, that I find it incredibly weird to ever speak to them in, or hear them speak the other.  Even though I know they're fluent in it. 

I bring this up because a particular friend of mine is a huge Americanophile.  He lived there as a kid and makes sure to expose himself to as much American culture as possible.  In all social settings, I've only heard him speak Philly accented English, to the point of pretending not to understand Hebrew so people will speak English to him, and he's a stickler for (American) English vocabulary, spelling and grammar.  That said, I went to visit him at work yesterday, and for the first time ever after knowing him for years, I saw him typing away in Hebrew and having Hebrew conversations.  He even spoke Hebrew to me while there were others around.  And... this was the absolute weirdest thing ever for me.  I'm still kind of in shock.

Am I a freak or does anyone strongly associate people they know with a particular language and feel uncomfortable speaking to them in another language or even feel weird hearing them speak it even tho both parties are fluent in it?