Im kind of confused as to your requirement. What is it you want, again? You want to develop your own scripting language, and the interpreter is written in C++? You must be very comfortable with parsing and the first steps of "compiling". By this I mean these steps (a few of the top of my head): syntax checking, creating lexemes, tokens, semantic checking, and finally interpreting or "running".
It is certainly a big task, but Im sure you will learn
a lot. So, to reiterate, starting this project and immediately going to "what language is the interpreter going to be written in" is probably not the best approach (in my opinion). If you arent comfortable with the things Ive mentioned above, I would start with those. A classic book is the "dragon book":
Dragon Book (computer science) - Wikipedia, the free encyclopedia. If I remember correctly, its not exactly directed to "interpreters" (in the sense of "non-compiled" languages), however, many of the concepts overlap.