You said it was fairly normal. It is not. Even a provisional google search will tell you only 3% of the global population speaks 4 languages or more. Yes, diaspora kids will have a disposition to speak multiple languages, but not many are as proficient as Lukaku. I'm not even doubting your anecdotal evidence, I'm just saying it isn't fairly normal, because, well, it isn't.
It's not a diaspora thing though. I'll give you an example for an african from a french colony that went at least to highschool it's not rare for them to speak decent english, french, the most common national dialect and a regional dialect. After that you have many that move to countries like Spain, Italy, Germany and add other languages. I also didn't even took into account the fact that both parents could have two different native languages or anyone that travelled to over african countries.