The Technovation curriculum is built around App Inventor and assumes that teams are using App Inventor to develop their app. Technovation does not provide resources for learning other programming languages. However, the Technovation curriculum teaches the fundamentals of computer science and can be used in conjunction with other resources to learn a different programming language.
Teams are more than welcome to find resources for other languages and develop their app in the language of their choosing. (Past teams have used languages like Java and Swift to develop their apps.) There is no restriction on languages, but teams must produce their app -- not their mentor(s) or others'.
If you have a question about using an app builder, check out this article.
* Please review the technical checklist and note the technical features that will need to be included in the team's code in order to receive full credit.