The majority of rusEfi code is written in very plain C++ ("C with classes"), with rusEfi console and some code generating
tools written in java.

Most of the needs are within the C/C++ firmware and automated testing. 

A good first contribution could be adding some automated testing into https://github.com/rusefi/rusefi/tree/master/unit_tests/tests

For current coverage report see https://rusefi.com/docs/unit_tests_coverage/

Testing coverage open tickets: https://github.com/rusefi/rusefi/labels/automated_testing

All open tickets: https://github.com/rusefi/rusefi/issues

See https://rusefi.com/wiki/index.php?title=Manual:Try_It for how to work on rusEfi code with zero or minimal hardware.