When did dutch become official written language of Netherlands?

Answer 1

i don't think that is clear. i think ever since the Netherlands were separated from flanders in 1586, dutch was the official language. this implies that you are talking about "modern dutch". of course, the Netherlands have one more language (though not official) called "frisian" which is linguistically situated between dutch and English.

for flanders the status of dutch varied. dutch (or flemish variants) were used throughout flanders and holland until the sixteenth century. flemish variants remained in use until 1830 when Belgium was established as a francophone state. dutch (flemish) then fell into disuse until the late nineteenth century when flemish authors started to mobilise politicians to recognise dutch (flemish) as an equal to french. this official recognition did not take place until after the second world war when it was made possible for all speakers of flemish to study in their own language from kindergarten right through to university.