FthinraKathi is a language that predates Surak, and is officially extinct. Luckily, T'Mar's mother has a thing for dead languages. So does Pylkau, it seems, who just does not like the idea of a Betazoid at the monastery at all.
It is also when T'Mar and Juliette reconcile, so it's not all Vulcans hating on Juliette. This is also a chance to flesh out T'Mar more, and show what lies beneath the cool Vulcan exterior. T'Mar is a novice with secrets.
The Vulcan Language Institute, linked above, was extremely helpful, and the history of the language structure really helped with the idea of a Vulcan that rose as a spacefaring culture long before the other species, but almost didn't survive the final test of any species -- having the power to destroy itself.