the tengku's lost language

The tengku are very good at assimilating into other cultures and can learn new languages very fast.  They will adapt to any culture but during the the Second Age they mostly attached themselves to elven nations and communities and during the the Third Age they have mostly attached themselves to human nations and communities.
In some ways, they were too good at assimilating to other cultures.  During the Second Age, most tengku spoke Elven more often than their own tongue only speaking their own tongue in private.  While tengku are highly literate compared to most other races, they almost never wrote in their own language.
Tengku made a point to teach their language to their young to keep it alive, but the language still declined over the Second Age.  Overtime from disuse the tengku probably lost 10-20% of their languages words via lack of use.
During the Second Unmaking, most tengku that survived only managed to survive by closely tying themselves to enclaves other other races.  When survival was the prime concern during the Second Unmaking and the Red Era, it was less of a priority to teach tengku chicks their mother tongue when teaching them Elven and Common  will help keep them alive.
During the current Feudal Era, the tengku work fiercely to teach their young their native language but by this point, only about 30% of their original words are left in their collective memory.
Their grammar is pretty much forgotten too, most tengku will splice their own language into the framework of Elven or Common grammar.

Geographical Distribution

The tengku have no true homeland and are spread throughout all of Scarterra, so their language (what is left of it) is spread throughout Scarterra.
The tengku still rarely put their language to writing, but after losing over half their language during the Second Unmaking, they have created books containing all the words they know and entrusting it to Masks and Keepers to preserve what is left of their language for posterity.

Phonetics

The tengku language, to an untrained observer is not especially different from a raven squawking.

Common Phrases

The most common words that remain include things that are both important and personal to the tengku:

-Terms of endearment for family and friends.
-Commercial and business terms
-Curse words and profanity
-Geographic descriptors and travel conditions
-Terms to describe non-tengku