The intermediate code generation is in a language called YAIL, which is a Scheme with additional macros that provide a clean interface for the blocks to translate into. In the companion, this code is interpreted but in APKs it is compiled directly to Java bytecode before running through the normal Android build process.
The point of the Java bridge research project was to produce a version of App Inventor that could generate Java code rather than Scheme. However, I don't believe the results of that project were ever open sourced.