Conlang
Also said as Constructed language, Fictional language
A language invented for a work, with its own sounds, grammar and words. Real ones are also given a history, with sound changes that split one old tongue into several new ones.
Most games do not need a full one, but a little goes a long way: shared roots across place names, or two dialects that are clearly related, can suggest a thousand years of history.
Say it
“We do not need a full conlang. Make 40 root words and two sound-change rules, so the northern and southern place names look related but clearly split a long time ago.”
Where you’ve seen it
Far Cry Primal's Wenja was built for the game by the linguists Andrew and Brenna Byrd, from reconstructed Proto-Indo-European, and the actors learned to perform in it. In Skyrim, the dragon shouts are words in the dragons' own language.
Not to be confused with cipher
A cipher swaps the letters of a real language, so a player can decode it back to English. A conlang is its own language, with its own words and rules.