Effectively nothing. 'Brand activation' is the English term, 'merkactivatie' is the Dutch one. Both describe the same thing: a live moment where your brand actively connects with the audience, usually in a public space or a busy spot.
In practice, 'brand activation' is used more often for larger campaigns with an international slant and pronounced brand positioning. 'Merkactivatie' tends to refer slightly more to Dutch, smaller-scale moments. In substance there's no difference.
Choose the term you feel most comfortable with. The method is the same.