Notaris

Flemish law and regional competency => best to use a Flemish notaire.

I'd probably agree with J here, if you're buying in Flanders, then you're probably better off getting a Flemish notaire.

As for languages, I won't bet on it, but I think you'd be hard pressed to find a notaire that doesn't speak English.

My friend bought a house in Wezembeek using a French notaire and they were very happy with her.

Your notaire can be anywhere. You can see where I live and my notary was in Antwerp. Wezembeek-Oppem is a Flemish commune but the population is predominantly francophone so drifters decision was fine. If buying in Flanders I would go for a Flemish notaire. Talk with them in English and make sure they will be happy to fully explain all the paperwork in English to you.

The specific rules and percentages for real estate differ by region, so your best bet is to choose a notaris based in Flanders. All the paperwork will be in Dutch (as is normal), but I'm sure most notarissen would be willing to explain the details in English.