I don’t think you are a bad parent at all OP. It’s a very sad situation for you both.
Is it possible he’s addicted to gaming because it’s a solace to him, masking his very real problem of having to deal with adult decisions ?
Is it possible he doesn’t engage with his studies because he’s actually very clever…maybe even gifted and yet doesn’t want to grow up and have to make those same adult decisions ?
Living an international life doesn’t always come with a guaranteed positive outcome for youngsters, in that life can be too easy for them. It’s a bubble kind of privileged life very often, where everything is provided and the cost of stuff doesn’t enter into their thinking. Maybe that’s not the case for your family but did he ever hear words like …….no you can’t go on that ski trip because we simply can’t afford it. ?
When he asks you …”how exactly are you going to make me do that, “ whether it be getting a job or cleaning his own bathroom, maybe he relies on the fact that you or his dad will always provide, and always pick up after him because he knows you love him. Maybe he is actually deeply ashamed of himself and goes back to gaming to distract himself from those terrible thoughts.
I can fully understand your desperation at wanting to help him now …to stop the lack of respect and bad language and dirty habits before they spiral down any further. It could well be a form of depression where he just can’t face adulthood and how to go from being in high school to setting his own course and providing for himself. It’s all too big a jump now for him to self motivate so he takes the easy very defensive way out, hurting those who love him.
I have no advice other than what has been offered already, but I can easily see how this situation can develop. I would gradually make him aware (without any big dramas if possible) that privileges such as wifi, clean laundry and 8 eggs a day come only with good manners (which after all, you know he is perfectly capable of) and aren’t to be expected without them. Drip feed that thought into his psyche whenever you can. But always be ready for an honest (not just lip service) heart to heart if he prefers.
No doubt he now gets an allowance rather than pocket money. I would keep it going for now but gradually make him aware it’s dependent on good manners and he loses a month next time he uses bad language against you or others in the household.
I would forget about his studies … just refuse to pay to enrol him in any more education.
In order to avoid him becoming reclusive which I’ve seen happen before in kids who shut themselves off because they just can’t take the next step, I would keep encouraging the gym, the rugby, the sailing and anything else so that he always stays connected to society.
Maybe it will be his peers in the rugby club who eventually get through to him and offer him a lifeline into the adult world. An easy job for just one day a week helping out might help him start back onto the right path.
Good luck OP.