Are all the cases during the 2 differents "rounds" the same difficulties, or do they get progressively harder ?

So at least in Germany the cases do not differ in difficulty. Each case interviewer will place a different focus. So it does depend on your strength and what case the interviewer chooses.

Okkkk thanks Martin !

Another question, maybe you'll think it's a bit dumb, but does "style" matter during the interview ? I mean, it's a suit and a propre "clean cut" hairstyle I assume ?

No worries.
So business attire is a must. But also consider what you feel comfortable in.

For my internship interview day I did not wear a tie. So it wasn't overly formal.

Ok, so no tie for me neither ;) Thanks for the tip Martin !

Also feel free to reach out to our French colleagues for that question. They are happy to give you local guidance on these kind of questions.

Yes, will do ! Thank you :)