Double Dutch is nonsense. At least, it sounds like nonsense: but of course that's a different thing.
Some forms of Double Dutch, like Tutnese, Ubbi dubbi and Izzle, for instance, are really ordinary English that's deliberately been made hard to understand. Sometimes this has been done for fun, and sometimes it's been done to fool your enemies (there's a history of African Americans using Double Dutch in this way).
Sometimes, however, the reason Double Dutch is hard to understand is because people are being unreasonable enough to speak a foreign language.
In the 1600s England and the Netherlands fought each other in a series of wars, and so to the English more or less everything Dutch became really annoying. That's when the term Double Dutch - a language even more baffling than Dutch itself - seems to have been made up.
Mind you, even undoubled Dutch can be baffling, because Dutch is not always, well, Dutch at all.
The famous Pennsylvania Dutch, for instance, speak (or used to speak) Deitsch, which is actually a sort of German.
Why is it called Dutch, then? Perhaps because of a confusion between Deitsch and Dutch, and perhaps because right up to the 1800s Dutch could mean anything from anywhere from Belgium to Austria.
If you wanted to distinguish between the Dutch and German languages then you called them Low German (which meant Dutch) or High German (which meant German). To make things even more confusing, it's only quite recently that Germany has existed as a country. And there's never been a Dutchland at all.
I'm not surprised.
In fact, do you know something? I think it might make more sense to talk, not about Double Dutch, but about Double German.
Word To Use Today: Dutch. This word comes from the Middle Dutch duutsch, from Old High German duit-isc, belonging to the people, from þeod nation. It first meant anyone who spoke a Germanic language, then it gradually narrowed down until it meant specifically to do with the Netherlands.