closer translation of dutch index.html

I think this is a better translation, closer to the original english. 

Also, I added the word 'valuta' because a 'betaalmiddel' has a slightly other meaning: It can mean 'currency' (as in 'wettig betaalmiddel'), but also 'payment system' (ex. 'pinnen') or 'instance of payment system' (ex. 'een checque').

In fact, I'm in favor of removing the word 'protocol' too as it has no meaning for an end user and maybe redundant with 'payment system' but we should discuss that in the english branch.