Well, a mod adds features, so theoretically, it should be possible to translate these features into another language, even if it is probably complicated like hell due to exceptions because programming languages work differently, so it would probably be a piece of code rewriting the mods with the same features for a different language and engine, before the game actually starts. And as that would take a while, it would probably save the results and use them the next time.
EDIT: Or it adds its own language and figures out how to convert it to the other languages.
They made it for java, that will not work with C++ for example, They made it in a stupidity of retardation class'splosion style, which will not work well with better designs. They are not using an intermediary language like Terra for code generation, or even lua for slow interpretation. The very design would fail even between MC versions due to client architecture changes in style. Their design is inherently flawed on a *multitude* of levels, what they want to do is possible sure, but not the way they have done it, not even for just MC for anything more than simple things like their example.
Side-partially-related note: GT's API tries to remove the mc-stupidity of class'splosion by making single monolithic reusable classes, it feels like it is trying to become component based but instead of doing it dynamically, which is far more powerful and very fast in both coding and execution time if done right, it instead is trying to 'bake' all components in to one massive main 'entity'. You really should finish your partial component design and go all the way.
P.S.: Still on phone, please excuse spelling, grammar, and terseness.