Teams can develop their app in any language of their choosing. There is no restriction on languages, but teams must produce their app -- not their mentor(s) or others'. To receive full points in the technical category teams need to be able to complete the code checklist and submit source code with the programming language they are using. The Technovation curriculum teaches the fundamentals of computer science using App Inventor but can be used in conjunction with other resources to learn a different programming language.
Teams may use publicly released libraries, example code, and tools, but teams must also develop original code. To be valid, every submission must include a source code file of the app.
You are allowed to use app builders and programming languages other than App Inventor. However, you need to be able to complete the code checklist and submit source code with the app builder you are using to receive full points in the technical category.