Important thing: The game is free.
In their document they used a lot of English names, so I think "open source" would be better than "wolne oprogramowanie", especially because it actually means "free software" (and that's something completly different) - https://pl.m.wikipedia.org/wiki/Wolne_oprogramowanie. We could also mention that this makes the possibility to discover how the game itself works when the students start learning real programming languages. It also opens the game e.g. for contributions from teachers who make interesting exercise levels.
In their document they used a lot of English names, so I think "open source" would be better than "wolne oprogramowanie", especially because it actually means "free software" (and that's something completly different) - https://pl.m.wikipedia.org/wiki/Wolne_oprogramowanie. We could also mention that this makes the possibility to discover how the game itself works when the students start learning real programming languages. It also opens the game e.g. for contributions from teachers who make interesting exercise levels.