In my setting I use Esperanto as an ancient common tongue and English as a modern one. That's because it's ridiculously easy to construct false etymologies to explain various features of the English language if you use Esperanto as a base.

Esperanto words also have somewhat guessable meanings if you know your Latin and Greek roots, even though the text is not comprehensible generally. So players can have hints at the meaning of a text without knowing what it really says.

When I need to obscure the meaning more, I mix up the words in a sentence. Because Esperanto has an accusative case, you can mix up the sentence order without loss of meaning. But it makes the sentence harder to read and obscures the relationship between the words.

Also, it sounds like an incomprehensible but distinctly European language when spoken. Players tell me it sounds like Spanish.