A classic is something which has been very widely judged over a long period of time as outstandingly good.
It is therefore total nonsense that all old black and white films are said to be classic. They aren't. It is also nonsense to say that a film was not considered classic on it's release. How can iot be judged over a long time the day after release?
It is also nonsense to say that all foreign films are said to be classics, because they plainly are not said to be that.
The people who say those sorts of thing ought instead to tell us which film they consider to be wrongly rated as classic or 5 star.
You might not like Casablanca, or The Seventh Seal, but at least you really ought to be able to see why they are considered as outstandingly good.
It is in fact not possible to explain simply why they are great films. Of course it is not just plot or subject matter. A person might look at a Rembrandt self portrait or Cezanne's apples and if they ask why they are supposed to be so good, all one can do is shrug and say that they just are, and suggest looking at a lot more stuff and then come back to them, perhaps then they will see.