> First, is there anything I should know about Dutch people. Are there any taboo ?
It's probably really similar to France.
> Do all of your teens really speak perfect English as I heard ?
> I was also told that concerts were pretty violent compared to France or Belgium
I have no idea where this comes from. Heavy metal (and similar) concerts often have a mosh pit, but it's consensual fun between adults. Same could hold for more 'traditional' Dutch acts such as Normaal, which would be more easily found at for instance the Zwarte Cross.
> Google translate told me diesel was "diesel" in dutch, it that what I'll see at the gas station ?
Yes. They have a black handle and often a black flap over the handle which reads 'Diesel'.
> Any songs that will be sung at the camping ? (surely it's not a french thing :) )
No idea :( There's probably a douche there who'll play Wonderwall on his shitty acoustic guitar the whole evening.
> Any words I should know like... what do you yell if you invite someone to drink with you ?
"You want something to drink?", or say "biertje?" it means "beer". People will know what's up.
> Speaking of which, what do Dutch people drink ? Beer then strong alcohol ? Only beer ?
Anything goes. At festivals, most people would probably stick to beer on the festival grounds. Maybe they have a bottle of hard liquor at the camping.
> And is there anything I could do that may appear rude to you ?
Calling 'Frisian' a dialect when it's actually a language.