Unfortunately, the stripes don't seem to make a huge amount of difference when the civilian side jobs come up. A Group Captain married to a friend of mine, so Royal Navy Captain/Army Colonel equivalent, had to take an A2 posting to get his foot in the door (effectively an A5 to an A2 drop).
Your dp needs to get his options lined up, as applying for the NATO jobs is a long and convoluted process, and there is little point in leaving one job in this climate before securing the next. Some of the jobs are only for 3 years as well.
There is a big language divide here. To the south of Brussels (roughly) is Wallonia, which is Francophone (Flemish not spoken); some of the communes within Brussels are Francophone; some are resolutely Vlaamse tal, and I live in a Flemish area. Luckily for me, I live in an area where there is a large international population, so most people will speak English, but in the Gemeente (commune offices) and the tax offices they will ONLY speak Flemish to you. Same goes for the tip.
It is unadvisable in certain Flemish areas to speak French; in a gemeente near me the Estate agents have been sent notices telling them to desist from displaying notices in French and English, they must only use Flemish or they will be fined.
Flemish is not impossible to learn, I did two years and can get by, and my ds is doing it for his second foreign language at school I found it hard as grammatically it is a lot like German with verbs at the end of the sentence, and it is very gutteral as well.