Ok, so I went to Brussels, went to ISTI's open day and checked out Marie Haps too, so here is the info I got:
In Brussels, apparently, it is illegal to select students through an entry test before starting a Master's program.
So at ISTI there are no entrance exams to get into the Master in interpreting and at Marie Haps, apparently, there is an entry test but they can't legally deny you access to the course, so even if you fail the test but still decide you want to take the Master, you can go ahead and do it.
Now the tricky bit you really need to know:
at ISTI they don't just accept you on the Master because you have a degree and can provide proof of your language skills or whatever.
They accept you on the basis of "equivalence", i.e. they look at all your academic studies up to now and basically want to see that in terms of content it is (very) similar to their Bachelor in multilingual communication (which includes language courses, some history, some economics/law possibly, can't remember, it's that sort of program anyway, not just language and literature).
They require the following documents from you if you want to apply:
-letter of motivation
-copy of your ID
-copy of School diploma
-copy of University diploma
-List of courses and grades of your university degree
-a description of every course you took at University. You can't write it yourself, they want it to come from your University (that could be your faculty's students' handbook with a list of all the courses available, for example...)
-proofs of your stays abroad
-proofs of work experience if you have it
Any documents, diplomas that are not in French, English or German need to be translated into French by a certified translator in Belgium.
I asked in the students’ secretariat what the selection criteria were, they told me it is all about equivalence, basically there is one person in an office who sits down, looks at your application and compares the descriptions of your university courses with the content of their bachelor in multilingual communication. I then talked to some students who told me it seems to be, indeed, a purely administrative process and themselves they had a degree from a University abroad (a bachelor) and had applied for the Master in interpreting but found themselves having to take the final year of the Bachelor in multilingual communication at ISTI because of this "equivalence" issue before being allowed to take the Master.
I also asked a professor (in one of the language departments, not interpretation) whether this whole "equivalence" process was an administrative or an academic one and whether an academic commission or administrative staff made the final decision on that... Well, I didn't really get a clear answer to that, other that the professor agreed that it was a very relevant question indeed, that Belgium was a bit strange in this respect and that equivalence issues seemed to be a plague that you just had to accept in academic life in Belgium in spite of the Bologna process...
Anyway... anyone planning to apply to ISTI, you are advised to submit your application early (April is a good time to start they told me) include all the documents they require and explain your language skills etc. in your CV and "letter of motivation" in such a way that a member of admin staff who may know little about interpreting can get it and see why everything you did qualifies you to take this Master in interpreting. If you can make your life and studies up to now look as similar as possible to their "Bachelor in multilingual communication" on paper, that should help you get in with less hassle.
As for the program at ISTI: from now on you will only be able to do ACC. You can, of course, attend extra classes if you whish but basically officially you can only take ACC. That is because otherwise the workload would be too high in terms of contact hours and, considering that many students go straight from the bachelor to the Master, they usually just have these two C languages. There used to be an option to add a retour or a C in the second year of the program but that is no longer available.
As for Marie-Haps, I don't know yet... I expect this whole equivalence business might be easier seeing that they do have an entry test ... but then it might not...
It all sounds a bit off-putting, slightly odd (admin staff deciding on the basis of course titles and description of your previous studies whether you can take the master in interpretation sounds a bit odd to me and entails all sorts of dodgy implications, too...) and complicated but might still be worth the hassle , seeing that Brussels is a top location to get internships and all sorts of opportunities to practice during your studies...