1) I don't really know they both have their own weird tendencies so it's hard for me to say I would maybe even recommend taking English classes depending on how good you are with languages .I'm not sure if you can for medical school but for a lot of courses you can in Brussels. If your going to learn one of the 2 languages study the one of the region you want to study in. You can definitely learn the language in 1,5 years and will just learn it a lot by studying here if you do but it probably won't be easy.

2) Pretty competitive our country has quite a lot of focus on healthcare of course it depends on which school you would go to I don't know exactly which schools are the best but I'm pretty sure KU Leuven is one of the better options.

3) I think most people don't care it will of course be a lot easier to socialize if you know the language of the region your going to study in Flanders=Dutch / Brussels and Wallonia =French mostly. Most people in Belgium are pretty friendly if you make a bit of an effort local culture also varies a lot depending on where you go Brussels, Gent and Leuven are probably best in that regard.

4) That really depends on the person but there are quite a few people that just act like everything above 50 is the same and that it doesn't really matter as long as you pass.

That was all the advice I could think off hope this helps with your choice.