The genius behind this claim (and he actually was a clever bloke when he wasn't trying to expound on the Bible) was Johannes Goropius Becanus, born in 1519. This polymath believed the Garden of Eden was located in Antwerp, and that the language of Eden was Dutch (specifically the Antwerp dialect).
Proof was abundant if you studied etymology. "According to Becanus, Adam apparently derived from the Dutch compound Haat-Dam (Dam-Against-Hate) and Eve is Eeuw-Vat (The-Eternal-Barrel)."
According to Dutch Israelites, the Dutch were one of the lost tribes of Israel, namely the Zebulun. After all, one of the children of Zebulun was called Helon, who gave his name to Holland. Some outlying Dutch fundamentalists still believe this...This identification of Zebulun with the Netherlands was later carried over into the British version. The Dutch could be graciously conceded their identity as the tribe of Zebulun as long as Anglo-Saxons could appropriate the more important tribes of Joseph (Ephraim and Manasseh) unto themselves. The Brits however only came to this amazing discovery after the Dutch, given that John Sadler, Oliver Cromwell's private secretary, was the first to indulge the fantasy. Sadler was born in 1615.
An Anglican scholar named C. T. Dimont summed it all up in 1933 - around the time a certain Herbert Armstrong uncritically adopted the whole kit and kaboodle - with the words, "It must be said quite clearly that British-Israel turns the Bible into a handbook of national megalomania".
All of which seems very quaint today, except for the fact that a number of apocalyptic sects that venerate Armstrong still actively promote British Israelism, albeit often tied in with a rabid American exceptionalism. National megalomania still accompanies the teaching.
I'm indebted for much of this information to the article by Khaled Diab, "Did Adam and Eve Speak Dutch", featured in Haaretz.
But still, I'm tempted to add, it's all Dutch to me.