i.e. there is a lot of commonality between programming languages just as there is between human languages, so training on one language would be beneficial to competency in other languages.
I assumed that is what was catered for with "even when controlling for the size of the training set".
I.e. assuming I am reading it right: That it is better to get the same data as 25% in 4 languages, than 100% in one language.