In general
Will I be all alone?
Yes. And No. Of course you are going to start this journey on your own. Nobody else than you can decide for you to start this adventure. I for example was really scared of not being able to connect with other students and being alone for the whole semester, but I tend to always overthink everything. Just keep in mind: Every single Erasmus student is feeling the same as you right now. everybody comes here on their own hoping to find friends, make great experiences, learn new things etc. so getting to know people is pretty easy. Also everybody is really open, so YES you come here alone, NO you will not be alone.
So can I just party for a whole semester?
I know that a lot of people perceive Erasmus as an excuse to ditch Uni in a student's home country to be able to party, drink and dance for a whole semester. However, for the most of us it is not like that. Of course, I passed my Erasmus in Belgium, the country of beer, and yes, you might go out a few times more often than you would be in your home country but not as excessively as most people would think because we still have to earn our credits (30 ECTS for me for example). Uni is still a big part of this experience because you still have to earn your grades.
So are the courses really so easy, that everyone is going to pass?
At my uni (EPHEC) there are two types of courses: 1. courses for only Erasmus students 2. COurses for Erasmus and regular students.
I would say that the Erasmus courses are a lot more chill in terms of delivery dates etc. but the grading is still not as easy. There are students who fail. There are students who just pass. No grades are being gifted just because you are Erasmus, sorry.
How will I manage to be separated from my family/friends for a whole semester?
Obviously it depends on the type of person you are but for the most people I´ve talked to it waas the same: In the first days when you don´t know anyone, it is hard. You will miss your family and friends. Then, when you meet all of the new people, you will start connecting and doing stuff with them. After a few weeks you will not think about your family and friends that much anymore and a few more weeks later you might even sometimes forget about them (in the best way possible) because you are just having the greatest time here. An then, when the end is coming near you are sad for leaving your new friends behind :(
Also, a semester is not that long. At the beginning you will think of all the time that is still left and five seconds later it is over and you´re asking yourself where the time has gone.
Especially in Brussels
I would not suggest coming here, or at least to Brussels if your only aim is improving your French skills, because almost everybody speaks perfect French, English, Flamish and often German too, so if they notice that French is not your first language, they will try and make it as easy for you and talk English to you. That´s obviously a nice gesture but also kind of hindering if you try and improve your franch skills.
The weather here is almost never nice. Meaning you should be aware that almost every day here is cloudy and foggy. Rain is the usual. Actually, in Brussels there is more rain on average than in the UK. So always bring your umbrella and don´t plan to only go out and do stuff on nice days because they are really rare.