First of all, welcome to Cemetech! Glad to have you here
Secondly, I would recommend using hybrid basic (which comes with Doors, however it is not out for the CE yet, so you'll have to wait) or Assembly rather than trying to create a new language. I'm not exactly sure how (or even if you can), but it involves some pretty advanced thinking.
However, you can kind of make another "language" by programming it in TI-Basic, but you would still be limited to TI-Basic commands. It wouldn't really be a new language, but what I mean is like you could make a program where you input your code, in a made up language. So in the new "language", you could do something like draw.Circle(X,Y,Radius) rather than the default Circle(), or you could make up a command that runs a certain routine. The user, for example, could type in the whole command, or you could have a "tab" option to insert the command it thinks your typing. It just depends on how you want to program the IDE.
Hope this helps. If you're wanting something else, I'm sure someone else can help you