In the early days of international trade, the Dutch and English were forever at each other's throats. As a result, the word 'Dutch' was frequently used in phrases where the people could be made fun of or belittled.
The idea behind 'double Dutch' was that just Dutch itself was hard enough to understand, so doubling it made it something utterly incomprehensible.
'Going Dutch' refers to a situation in which a man has invited a woman out on a date and then expects her to pay her half of any bill arising. In other words, the Dutch are mean and far from gentlemanly.
'Dutch courage' was bravery that you could only experience by first getting drunk...in other words, the Dutch were really cowards.