[wkat4242]

It's not really secure as such. It's a simple replacement cipher theoretically speaking. An adversary with enough time will easily be able to figure out what each word means by association of known plaintext.

However it sounds like they were mainly used in heavy combat conditions where the enemy didn't have recording equipment for later analysis. So in that scenario specifically (but in that alone) it was pretty secure.

[withinboredom]

Yes, for the parts that needed to be distinctly translated from English, it was a cipher. However, it IS a language, with it's own grammar and vocabulary. They could have an entire conversation with none of the cipher bits...

If you've ever gone to a foreign country, you know you will have zero idea what anyone is saying for quite a long time. They knew as long as they kept the messages short enough, nobody was going to learn it from immersion.

There are still ancient languages that nobody have deciphered, despite having copious samples to choose from. I don't think it's as simple as you're making it out to be.