IMPORTANT !!! Everyone. For 'Spelling Check', download 'Woordenboek' from this page https://addons.mozilla.org/en-US/firefox/language-tools/
This language pack cannot be used for spelling !
I was confused too.

Cannot choose dutch language for spell-check after installing the language pack. What does it do?

It doesn't work! At all!!

tot nu toe is voor mij firefox de beste

Zeer tevreden.

The language add on is installed but it does not work: Dutch language is not possible to switch to.

Trying to get the Dutch language pack added, but after 30 minutes of trying it has not activated

Indispensable en Belgique ... ;-)))