Calantic

Calantic is the ancient language of the Calanti people, rumored to be the most optimized humanoid language for spellcasting.

Description

Calantic has a massive 50 letter alphabet, and it is spoken both verbally and somatically. Most of Calantic has been forgotten, as the civilization who spoke it was terminated by the gods a very long time ago. Some small sayings still exist with vague translations over into common.
Scholars will dedicate lifetimes in order to decode the few ancient texts that they have recovered. The Calanti people were history's greatest spellcasters, and by understanding the language, scholars believe that their spells will become more empowered. There are also scrolls written in Calanti that supposedly give access to great forgotten spells.
While Calantic is very closely related to magic and spellcasting, there is nothing inherently magic about it. It can be spoken just like any other language. Many of the Calanti people weren't spellcasters at all. That being said, any spell spoken in Calantic is slightly stronger than it would be spoken in common.

Writing

Part of what makes Calantic so mysterious and difficult to learn is the difference between spoken word and written word. Calantic speaking requires hand movements, while the writing notation can't show that. That is, not that current society knows of. I know the secret, but I'm not going to tell until my players find out. :)
Another reason for the language's mystery is the lack of primary sources to research. All of the writing is lost somewhere in the Terralesque Desert, and nobody ever goes into the Terralesque Desert. It's just not worth the risk.

Controversy

There is a significant population in Distotia who are against the research of Calantic. The Calanti were cursed by the gods due to their hunger for more magical power. People now worry that by learning Calantic, leaders will anger the gods once again, and that they will suffer the wraith of Distotia's pantheon.