This is the exception that proves the rule - a normal rule of thumb is that any quote attributed to Einstein is likely to be misattributed.
In this case, Albert Einstein really did say something to this effect which was published as part of an interview with Einstein in the Saturday Evening Post of October 1929 by George Sylvester Viereck.
The interviewer quoted him, and wrote it down in English (Einstein would probably have made the original comment in German as I gather his command of German was far superior to his command of English). The source for the quote is the translated version in English.
The exact quotation in the original language has not been preserved. Here is the translation that was published by the interviewer, Viereck:
Viereck: "Do you look upon yourself as a German or as a Jew?"
Einstein: "It is quite possible, to be both. I look upon myself as a man. Nationalism is an infantile disease. It is the measles of mankind."