I don't see on Steam or on DTG site any notice about polish language version. This is already second DLC (after West Somerset Railway) without polish support. Additionally after changing TSW:CSX to TSW on Steam, link to a instruction is only to english version. Did you guys abandon polish support?
Have you sent a support ticket to DTG? I've found that doing so is a more certain way of receiving a courteous and serious reply than in this forum.
I would say, this isn't a case for the ticket, regarding their releasing plans. That would be just bad, but of course we can no one force to do things, they don't want. We can only vote with our wallets....
Every route is available in Polish except WSR and now RSN. What's more interesting - in English manual for RSN is mentioned Polish localization (on page 33).