I've given this some thought and Polish is no more complicated than most other Slavic languages, with maybe the exception of Bulgarian. They all share common characteristics of grammar and vocabulary. Standard Russian may be the greatest deviation from the original Slavic tongue because Moscow was pretty much on edge of the Slavic world when the language was developing. Naturally, I can't say which is the most difficult language to master because I have not been exposed to most of the languages, but the one that seems extremely difficult is Arabic. First it is highly declined and conjugated like the Slavic languages, but added to that it is massively idiomatic, so you have to learn all the nuances of their expresssions to speak it fluently. In this it's a little like English, whose grammar is very straightforward but whose idiom is also massive.
The most challenging languages for me to pronounce are Georgian and the Greenlandic variety of Innuit.
Georgian and some small Caucasian area relatives seems to be an isolate like Basque is, unrelated to any other large language families. I don't think it's turkic or altaic. I have heard it spoken but only by some Armenians so I can't say whether I have the full flavor of it.
A while ago, in pre-Internet times, I read somewhere the Polish language was used by Israeli pilots during the Six-Day War of 1967, and it also was, what Egyptians thought, an unbreakable military code.
A completely unknown language--except to its speakers--would indeed be an unbreakable code if it was unrelated to any known existing languages from which its structure and vocabulary could be inferred. If you could reconstruct Etruscan, you'd have yourself a perfect code. The Navajo tongue was chosen I think because the Japanese probably didn't have any Navajo speakers or even any linguists who had knowledge of it (I could be wrong about that, though) and there are quite a few Navajo speakers. Even if the Japanese had some linguists who could figure this out, it would be hard to break because Navajo was used on the spot in combat and its messages needed to be understood only in the moment. By the time a linguist got to it and tranlated it and sent it back, it would be too late.