"Our car leaves on Monday."
Translation:Nasz samochód odjeżdża w poniedziałek.
No. Firstly, you left out the "w" in "w poniedziałek", but most importantly, "swój" doesn't make any sense here.
"Marek myje swój samochód" = "Marek washes his (his own) car". Marek is a subject of the sentence and the car belongs to Marek.
"Swój samochód" here would mean that the car... belongs to itself? The car's car... that does not make any sense in this sentence, as the car is the subject here.
It actually exists in several languages, although I believe Polish is the only official language of any country (I put the Latin versions of Belarussian and Ukrainian aside) where this letter exists. I guess that would make Navajo the second most known language which uses it.
And by "garage" you mean the place where they fix cars, right?
No, this isn't that "My car is in the garage but it should be ready Monday and leave then". This is "leaves" as in "departs".
Which we decided is a pretty strange thing to say, in both English and Polish, actually. Probably only works in some carpooling contexts, e.g. when you arranged with some stranger who's going to drive from Warsaw to Kraków on Monday that you're going to pay half the gas money and you can go with him. The sentence would feel a lot more natural if it was about a bus or a train.
We're removing this sentence.