Yeoli

The secret language of the fauns is unique among languages. It has no written or spoken form, but instead consists of motions and fluctuations in temperature. The fiery nature of the fauns allows them to control the temperature of their bodies and the air immediately around them, while their unique vision allows them to detect very subtle variations of such. In this way they are able to communicate surprisingly complex concepts via an outwardly very simple sign language.
Due to the biological factors required to adequately communicate in yeoli there are a vanishingly few who understand the language outside the fauns, though there are some. Elves are the most likely outsiders to pick up the technique, leveraging their close ties to their feykith cousins and an aptitude for magic to bridge the gap. Other mages have learned means to engage with yeoli, but their usage tends to be very rudimentary compared to the feyn. 
The only other group to see any significant adoption of the language is among the Adventi, some of which can detect similar temperature changes in the water. This usage was created intentionally to replicate the benefits of salient communication they fauns enjoy rather that out of a desire to communicate with the fauns themselves, as fauns rarely if ever find themselves under the seas as the environment is so antithetical to their nature. There are some fauns who interact with Adventi in ports, but ironically these interactions happening outside the water neutralizes the Adventi ability to detect the temperature fluctuations necessary to understand.
Yeoli is considered a secret language mostly due to its extremely limited application, not due to some intentional obfuscation on the part of any group of fauns or feykith in general. Indeed, most are more than willing to share the language with any who show and interest and the capability to use it.