Hello, Manuela!
In order to find questions asked for the FIT part in the Spanish offices of BCG, I would recommend going through Glassdoor and evaluating the information written under interviews.
To save you same work, here it is: https://www.glassdoor.com/Interview/Boston-Consulting-Group-Madrid-Interview-Questions-EI_IE3879.0,23_IL.24,30_IM1030.htm
If you want to go the extra mile, I would (1) segment the information per role (2) evaluate frequency of questions.
Good luck :)