One word: i18n. JJ2 has a list of the official level filenames, and if it thinks you're playing one of those levels, it delivers you text from the .j2s file of your choice. That's how you can play the same official level in two different languages and see two different sets of text strings. However, it checks the filename sloppily and if you're playing, say, train.j2l, it'll assume you're playing trainer.j2l.