Well, obviously, we'd rather you didn't fork MTA - but you can if you want to. You need to release the code though, if you make it publicly available.
Why are you doing it? I don't see what you gain from splintering the community? We're more than happy to accept patches to add features that are missing from MTA (including translating it to Polish, if you so desire).
If all you're trying to do is take credit for last 8 years of work we've put into MTA, then we'll take a pretty dim view of it, even if the license allows it.