Sperethiel

Sperethiel is the language of the Elves, brought back into the world during their emergence. It bears no connection to other languages, as it is supposed to be directly from the Fourth World, which makes learning the language for non-elves difficult. Sperethiel is the official language for each of the elvish nations, including Tir Na Nog and Tir Tairngire.
The existence of Sperethiel is one of the bigger pieces of evidence the Immortal Elves do indeed exist. The theory goes that the Immortal Elves carried the language through the Fifth World and re-introduced it during the Sixth World. This theory hinges on the idea that Immortal Elves were able to gain positions of power during the early Sixth World, in order to spread the language. Although this isn't impossible, most experts believe that such positions would expose the Immortal Elves too the public too much for them to still remain hidden.