For now, Python... but I did a lot of C++ and Java at my old job. I also used to co-teach a course in C# when I was in grad school.

I'm mostly prototyping things in Python since it's one of the easier languages to prototype with.

Once prototyping is done, everything will be done in Java... but the prototyping will take about a year, honestly. (We need to have a proof of concept and have it approved before we can go ahead and program it all in Java.)