A Germanic language invented by Jews in Germany. After being blamed for the black death, the Jews were forced out of Germany, spreading through Eastern Europe and taking their middle-high period German language with them and mixing in a few Hebrew words, such as:
Ganif (thief) from Hebrew (Ganav)
Shikker (drunk) from Hebrew (Shikur)
Mishpoyche (family) from Hebrew (Mishpacha)
The language is written, curiously enough, using the Hebrew alphabet. As it once almost completely resembled German, the language evolved in Germany while the Jews were somewhere else, and therefore the language is still basically middle-high German.
Yiddish was the mother tongue of millions people before the Holocaust