Today is the Belgium national holiday, hence I'm not supposed to do work.
So, after the past glorious week where Electricien Belgium started demanding their payment for the work done and their purchases, today I received a message from them that they will not hesitate to take our builder to court. After an initial panic (because the message was not clear to whom the court bit was addressed to), we've decided not to take any action (i.e. reply). We could not reply anything useful anyway, because they are contracted by the constructor (so, we can't really decide anything).
They have sent a message to the constructor too with an ultimatum for payment and a remark that the translator was disrespectful.
We'll see what the next days will bring...