I am Belgian (although living abroad). Languages in Belgium:
51% Dutch speaking
48% French speaking
1% German or other
Most people from any side of the country speak English very well to relatively well. Most Dutch speakers know a decent amount of French also, and the number of French speakers understanding Dutch is also growing. Note that because of political reasons, some Flemish people will refuse to accept French language terms but that is irrelevant for you I guess because the divide French-Dutch is more or less in balance.
English keywords, if they are good, do relatively well in Belgium, several big sites use English words for their domain.