For the 6th Speedlang Challenge, everyone received tablets in undeciphered languages and had to decipher them (a.k.a. construct the language it was in). Zethar, unfortunately, got a regular old Akkadian tablet, but the rest of us had some new languages!
These languages had to have a phonemic distinction get neutralized in some context, some kind of suprafixation, sound symbolism, a factivity distinction in their complement clauses, and an unusual agreement phenomenon.
You can read the full requirements here.