Wiktionary
extinct language
n. A dead language.
Wikipedia
Extinct language
An extinct language is a language that no longer has any speakers, or that is no longer in current use. Extinct languages are sometimes contrasted with dead languages, which are still known and used in special contexts in written form, but not as ordinary spoken languages for everyday communication. However, language extinction and language death are often equated.
Usage examples of "extinct language".
He cursed in a whisper, using language no fourteen-year-old ought to know: not just because it was obscene, but because it was obscene in an extinct language.
From the language I heard in your songs, which is now an extinct language to our people, we presume they were on one of our ships.