Apache UIMA is one of the building blocks of Watson -- https://cwiki.apache.org/UIMA/powered-by-apache-uima.html And UIMA is written in Java and C++.

I was curious about the same thing - did they ever mention what languages were used in the documentary?

The screenshots looked like Eclipse with Java. The brief snippets I could make out certainly looked like they had to do with evaluating rules and scoring judgment, but that could have easily been stock footage.

I didn't hear any languages mentioned. If it was only stock footage, they did a good job making up source code.