I see a lot of wrong infos here on my native country ... About the language: the ratio Dutch-French (it's Dutch really, only Flemish patriots will claim Flemish is a language, it's just a dialect of Dutch just like an American and an Englishman speak the same language with a few local differences) is about 50-50. 51% or 52% speaks Dutch, about 48% speaks French, and a minority of less than 1% speaks German. German is however an official language of Belgium and the German speaking minority even has its own parliament. Those are the three official languages of Belgium, however due to the country being quite multicultural a lot of other languages are spoken, we have a huge number of Turkish, Moroccan, Eastern European immigrants for example.
Note as well that the majority of Flemish people speak at least basic French and a fair number of them speaks it fluently. In the Walloon part of the country the number of people speaking Dutch is not as high but it is growing as well. So de facto many are bilingual to a decent extent.
The vast majority of Belgians speaks English very fluently or at least at a decent level. Hence, English keywords with .be should be no issue I think.
.be is very popular, in Belgium it is on par with .com. The last years it has risen in popularity. Also note that DNS.be, the registry, is doing a great job and hence was chosen to organise the launch of the .eu extention (even when they are not the registry for .eu, DNS.be was very much involved in the setup of EurID and the preparations for the launch of the extention). As the Belgian domains are rather cheap (especially when registering with a Belgium or EU based registrar) and as most Belgians speak multiple languages, I'd say it's quite a good extention to look at for investment. Dito for our neighbours Holland, where .nl is even overpowering .com as far as I know.
And no, I am not just bragging but just citing basic facts. I am not a patriottic Belgian at all, just citing the fact that .be is very widely used.
PS: within Brussels, which is de jure bilingual Dutch-French, 80 to 90% are French speakers. I'd even say there will be more Turkish speakers than Dutch speakers residing in Brussels. A lot of Flemish people do work in Brussels though so it's not like Dutch has vanished totally in the capital.