What languages do the Ascians speak?

[Cleretic]

It's Meracydian. You just can't tell, because you've never met a Meracydian. Want to prove me wrong? Find one!

More seriously, it's one of the relics of the original Ascian concept that just got thrown away in the revamp because it was no longer worth having, and really shouldn't be thought of any further than that. You want an 'in-universe' answer, maybe it's a specific Convocation/Ascian language; they develop a language that nobody else can understand, so that way they can talk without fear of being overheard and worked against or raising a panic. We do know that the Ascians had secret facilities and operations, after all, and that two of the examples we see are specific to Lahabrea (both Pandaemonium, and the End of Days relics being researched in Akadaemia).

Or maybe that's just how Ancient Talk actually does sound like when coming from mortal tongues and into mortal ears without any magic or superhumans around (well, superhumans beyond the usual level for the setting).

[UkcsAlias]

they develop a language that nobody else can understand, so that way they can talk without fear of being overheard and worked against or raising a panic.

Ascian 1337speak

It can still make perfect sense towards those who know how to speak it and what the words mean in it. But for others it sounds like a garbled mess.