legal principles
I know that in Switzerland a court will look at the time the bond lasts which has to be 'reasonable'. That is something between 1 and 3 years. Then an employee can only be bonded for training that is not considered training only valuable with that company. So you can be bonded for the cost of a type rating because that is something you can take with you if you leave the company providing it. But you can not be bonded for the cost of the company introduction course, since that course has no value outside of that company. Then they can not bond you for an amount that is not in line with the actual training cost.
A court will accept that you as a job seeker, specially if you have no job are at a disadvantage, and companies are not allowed to take advantage of your situation. That can be a reason that even so you signed under a bonding contract not every part of it is considered legal.
Knowing all this, the best strategy is to talk to the company, and cut a deal instead of taking them to court. If they feel that you are informed about the legal situation, you will have some leverage in the negotiations.
I assume that similar principles will be applicable in other European countries.