I've heard that if you speak multiple languages with about equal fluency (which I do not) the real test to see which is your primary language is to figure out what language you dream in.
Related to that, for developers, are the habits that you form from years of using a particular language. Here I'm speaking of what you normally code in. For example, the VB native speaker might think "dim so-and-so-variable as this" and have to flip it around in his/her head when typing a C# program. The C#/C++/C/java person comes at it from the other direction.
Me, I've grown up with VB. I've been working with it since VB 3.0. Early in my career I worked for a VB add-ons catalog vendor. I'm all VB all the time. (please hold the debates about whether VB.NET is really VB)
Anyway, this is all amusing to me because today marks the first time that I actually added a semicolon to a line of VB code. Usually when I'm coding in C# I'll forget to add them and get compiler errors like "expected ;" (which raises another question - if you were expecting a semicolon why didn't you just add it for me). This time I got one the other way, and I just sat back and laughed. Can't wait to see if my dreams tonight are case sensitive.