I remember this from a linguistics professor I had in graduate school about 30 years ago. He named the source, but I can't remember.
אַ שפּראַך איז אַ דיאַלעקט מיט אַן אַרמיי און פֿלאָט
a shprakh iz a dialekt mit an armey un flot
- Max Weinreich
As WavesWashSands says, the earliest known published source is Weinreich's article:
- דער ייִוואָ און די פּראָבלעמען פֿון אונדזער צײַט
Der YIVO un di problemen fun undzer tsayt1
originally presented as a speech on 5 January 1945 at the annual YIVO conference. Weinreich did not give an English version.
In the article, Weinreich presents this statement as a remark of an auditor at a lecture series given between 13 December 1943 and 12 June 1944:
A teacher at a Bronx high school once appeared among the auditors. He had come to America as a child and the entire time had never heard that Yiddish had a history and could also serve for higher matters.... Once after a lecture he approached me and asked, "What is the difference between a dialect and language?" I thought that the maskilic contempt had affected him, and tried to lead him to the right path, but he interrupted me: "I know that, but I will give you a better definition. A language is a dialect with an army and navy." From that very time I made sure to remember that I must convey this wonderful formulation of the social plight of Yiddish to a large audience.
Some scholars believe that Antoine Meillet had earlier said that a language is a dialect with an army, but there is no contemporary documentation of this.
Jean Laponce suggested in 2004 that Hubert Lyautey (1854–1934) may have originated the phrase at a meeting of the Académie française, and proposed to call it the "Loi de Lyautey" 'Lyautey's law'. But again there is no good evidence for this.
Randolph Quirk adapted the definition to "A language is a dialect with an army and a flag" (adding a defense policy and a national airline).
1. "The YIVO Faces the Post-War World" (lit. "The YIVO and the problems of our time"),
from "YIVO Bleter (vol. 25 nr. 1)" (in Yiddish). Jan–Feb 1945.
Max Weinreich is usually said to be the person who created the quip, but according to the Wikipedia entry on him, he was actually quoting someone else.