Actually, we say other things in English too, at least in AE, in my experience, at least some of the time.
'Margin' sounds foreign to me, possibly BE, or even Spanish? Around here we say the side of the road, or the curb in a city, or the shoulder (lane) on a highway.
'Crowd the center line' is understandable, but I'm not sure it's what would have first occurred to me if I were describing it myself. I would picture that more as describing someone who's impatient to pass, on a two-lane highway -- staying close to the line but not going over it (which would risk a head-on collision with oncoming traffic).
Just as general descriptions of an incompetent or unsafe driver, I would be more inclined to say 'can't stay / can't keep the car inside / in the center of the lane,' 'can't track straight (inside the lane),' 'drifts (wavers, wobbles, etc.) out of his/her lane,' 'drifts over the center line / onto the shoulder,' etc.
Or even more generally, referring also to parking, 'doesn't know where his/her wheels are' -- a problem for inexperienced drivers, but also for those changing from left- to right-hand drive or vice versa.
And yes, we also say 'can't make up his/her mind (which lane he/she is in / wants to drive in).'
'Crowd' could also be used in a related but slightly different sense. When I saw the thread title, I pictured one car pushing into the space of another car, coming dangerously close as if harassing or trying to push the second car off the road; or lanes that are too narrow and too close together, as in overcrowded cities in developing countries.
Not sure if any of that is any use to you, wupper, sorry; but perhaps it might be helpful to someone comparing a range of options in both languages.