This morning I got a phone call from Electricien Belgium that made things clear: Electricien Belgium were not happy of the delayed payment and threatened again to take the constructor to court because:
This is still Europe, not Africa and they have picked the wrong person to mess with.
Unfortunately, they were barking at the wrong tree, because they were subcontracted by the constructor and I could not do anything besides forwarding their message. We did forward the message to the hierarchy at the constructor's side.
The site manager replied to Electricien Belgium with a very confusing message, that they need to take into account my request... which was about him to make sure electricians don't perform work outside the contract.
Now, the constructor will have to decide the next course of action. I can only hope this gets resolved somewhat quickly.