"Are Vlieland and Texel all the wadden islands?"
Translation:Zijn Vlieland en Texel alle waddeneilanden?
I have the same question, but would word it slightly differently, i.e: "How do you differentiate between: 'Are they all Wadden Islands?', and: "Are they all (of) the Wadden Islands?'" I suppose when there are only two, it simplifies things, because you probably would not use "alle" to refer to just two of anything. But if there are more than two, it is not so obvious whether you are asking: "Are they the complete set?", or: "Are they all part of the set?"
"Are they all (of) the Wadden Islands?" as in they belong to the islands, (but there may be more,) would be "Zijn het beide Waddeneilanden?'' or when referring to more than two: "Zijn het allemaal Waddeneilanden?". In this specific case it can only mean "Are they all the islands" as in "the only two islands" (no more).
The Waddenislands are Texel, Vlieland, Terschelling, Ameland, Schiermonnikoog, Rottermerplaat and Rottumeroog. The last two are uninhabited. Texel is the biggest, Schiermonnikoog the smallest, and Terschelling is my favorite (but opinions differ).
And there are the German and Danish Wadden islands. :) BTW Waddeneilanden should get a capital letter both in Dutch and English, since the geographical place is meant here, not the general term wad for fordable/wadable place.