I'm sorry. I should have been quite clearer.
I should have written that "auto" is not just used in compound words here.
For example, one generally does not go looking for "car" parts but for "auto" parts, "auto" or even "automobile" insurance (sometimes even "vehicle insurance"). While "car" is used quite a lot it is not done exclusively, in quite a lot of circumstances not at all and "auto" as a single word is still used frequently. (At least in the US)
Not the least because there is in fact a significant number of people learning English here. Or improving their English while learning Czech. Some exercises are even common with the course of the English language for the speakers of Czech.
It is not my program and not my rules in any way. I created neither of them nor did I significantly participated in creating the Czech course. If you mean Duolingo, then I am not a Duolingo staff although I do represent it when I moderate the forum.
You're right that "hezký" usually means "pretty" and that's why "pretty car" is accepted. However, note that there's a difference between "hezký/krásný" as there is between "pretty/beautiful". As to the "official" translation - if you comment on someone's car saying "hezké auto", it'd generally be best translated as "nice car".