App Inventor has no libraries of routines; consequently it does not have an internal spell checker. Here is what the textbox can do on its own
Since you are new to AI2, here are some resources to help you learn to use the AI2 tools A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook
http://www.appinventor.org/book2 ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.The article provides insight to some of the capabilities of AI2 which is an entry level Android compiler designed for 'ease of use' and not capabilities of the professional compilers like Eclipse or Android Studio.