I see a lot of folks asking this lately. The full official English translation of 'Mo Dao Zu Shi' is out there now, put out by Seven Seas Entertainment. You can grab the paperback volumes or the ebooks from the usual suspects like Amazon, Barnes & Noble, or Bookshop.org. That's the safest route.
But look, if you're asking for the PL translation—that's Polish, right?—the waters get a bit murkier. There isn't an official, licensed Polish edition of the novels that I'm aware of. The fandom for danmei is huge globally, and a lot of the initial access came from fan translations. For Polish, you'd likely be looking at those old fan-translated chapters that were floating around on sites like Wattpad or specific blogs run by Polish fans. A lot of those got taken down when the official English license was announced, as a show of support to the author.
My two cents? Hunting for those scattered Polish chapters now is probably more trouble than it's worth. The fan translations were a labor of love but were often incomplete and varied wildly in quality. The official English version is beautifully done, incredibly consistent, and you're directly supporting MXTX. Maybe consider giving the English a shot, even if it's not your first language? The story's complexity really shines through in that translation.