Maybe it is a correct phrase in Swedish, but in English one should always define the subject. One is talking here of specific cars, 'the' American cars. Who/which/what - the car. The car is good. Is the car good? The American car is good. American is an adjective and car/s is a countable noun. 'American car is good' is an incomplete phrase.
The reason the subject is indefinite here in both languages is that it's a general statement. We're talking about American cars as a category, not about some specific American cars (for that, we would have used the definite form). There are some differences in how definiteness works in Swedish vs English, but in this case it works the same.
I agree with the other replies to this.. I'll add some more explanation: The plural is being used here to form a general statement (as Arnauti noted). The generalizations "Most American cars are bad" or "All American cars are bad" --> "American cars are bad". We'd understand the question "Are American cars bad?" as equal to: "Are almost all American cars bad"
Nonetheless, I greatly respect that you are being careful about this. Many Slavic language speakers I've met often miss articles when needed. (just guessing from your username)
Thanks for the comment. Yes, my native language is part of the Slavonic group. Sometimes the use of the articles and especially the way they are attached to words (via suffices) is very similar to the Swedish, but there are differences too. Like the rule that the nouns have either to be defined or undefined with no exception of missing the articulation and another rule requiring all the objects to be in their definite form. So this regulation obviously makes me confused at times with other languages. :)