She could have been talking double Dutch for all we understood of it.
Does this mean:
We may have understand some of it, but then again, she was talking nonsense so I don't know?
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Double Dutch means 'gabble', 'unintelligible speech'.
For all we understood of it means literally 'to the entire extent that we understood it' and implies that in fact we understood none of it.
Put the two together and they yield "Perhaps it meant something, but for us it might as well have been in a foreign language, since we understood none of it."
Double Dutch is an American game made popular in urban areas. It is a a way of skipping rope where 2 ropes are used at the same time - spun in opposite directions.
It is used in your example only because the word "Dutch" is English for the language spoken in The Netherlands - So, it is playing with the idea that if "Dutch" is a foreign language that is not understood, "Double Dutch" must be:
twice as confusing/nonsense,like the rhymes often used to keep rhythm while playing this game.