יידיש
Yiddish
Lower East Side +1Yiddish, the principal mother tongue of European (or Ashkenazi) Jews, has likely been spoken in the city since the 17th century, but large waves of speakers first arrived in the 19th century: initially German Jews, some of whom apparently still used now largely moribund Western Yiddish, and later large numbers of speakers of all varieties of Eastern Yiddish from across Yiddish-speaking Eastern Europe. By the early 20th century, New York City had become the largest Yiddish-speaking center in the world, with mass media, theater, and other institutions to match.
W
hen the Lower East Side became one of the most densely populated places on the planet in the early 20th century, it also became one of the world's major Yiddish-speaking centers. Following German (including German-Jewish) settlement in the mid-19th century, Yiddish speakers from across Central and Eastern Europe formed a number of distinct zones (for Jews from the Russian Empire, Galician Jews, Hungarian Jews etc) within the Lower East Side. Yiddish served as the common language, however, and was soon put to uses that were often restricted in Europe. Second Avenue became the world center of Yiddish theater, a massive Yiddish press and many schools of Yiddish literature flourished, and even a labor movement grew up around the language.
Note that the language above may be used throughout the New York area — this is just one significant site.