Apple Intelligence on MacBook Air in Belgium

Dutch is coming for Apple Intelligence, it's just not available yet.

As per the support document I linked to above, and the Press release published on September 9th.

New Apple Intelligence features are available today - Apple

It still states coming soon. Not that it would be coming with macOS Tahoe on initial release:

Additionally, Apple Intelligence features will be coming soon to eight more languages: Danish, Dutch, Norwegian, Portuguese (Portugal), Swedish, Turkish, Chinese (traditional), and Vietnamese.

Go to Settings ➜ Apple Intelligence & Siri and turn it on.

Make sure your display language and Siri language are set to a supported language.

click here ➜ How to get Apple Intelligence - Apple Support