Hi. Welcome to Flarum!
PeterKurvers We need a forum with good accessibility for people with all kind of disabilities.
We have not received a lot of feedback in that regard so it's hard to say whether the software meets your requirements in its current form. However if you notice any accessibility issue we will be happy to investigate and try to improve.
Could you give more details on what you'd like ?
PeterKurvers Users must be able to choose email notification or not.
You can choose whether you want web and/or email for each kind of notification.
PeterKurvers There must be a ReadSpeaker which can read per message, and can also read only the most recent message.
You can most likely get this developed as an extension.
Flarum ships with English out of the box but we have third-party Dutch language packs available. We also have quite a few Dutch users here. Translating additional untranslated text is also easily done via a custom language pack or the web-based Linguist extension.
Flarum is known for its extensibility. If the features you need aren't part of Flarum out of the box or have not been made available yet, you can most likely get them developed for you by a freelance developer (or your own team).
EDIT: oops, I took too long to answer 😇 I didn't see the other replies before posting.