I found this explanation:
(source:
http://theedexperience.wordpress.com/2011/12/13/double-dutch/)
"In spite of all appearances to the contrary, this phrase - meaning, 'nonsense', or 'gibberish' - does not seem to have originally been spoken with the Dutch in mind at all. Rather it was once intended as a slur against the Germans, those rascally scapegoats of the 20th Century, although my research suggests that even at the time (late 1700s - early 1800s), this little detail might not have been apparent to your average slurrer.
'Dutch' was originally the generic name for both Germans and, as they were formally called, Hollanders. High Dutch was the language of southern Germany and Low Dutch the language of The Netherlands. I have been at least partially persuaded that 'Double Dutch' is in fact a synonym for High Dutch and as such involves the Germans and not the Dutch.
In the way of so many of the greatest idioms, 'Double Dutch' originated from sailors. Indeed, the first recorded usage of the phrase, and many subsequent usages, refer(s) to Double Dutch in terms of the winding of rope. The indication here is that 'Double Dutch' is the linguistic equivalent of a badly coiled rope. Those sailors really knew how to make a man feel small.
There are a host of phrases in English that include the word 'Dutch'. Given the close proximity of the two countries, their proud maritime traditions and their long standing trading and military rivalries, this is hardly surprising."
