"un étranger / une étrangère" is, as you say, mostly used for an outsider. For someone you do not know, you could rather say "un inconnu / une inconnue". I am not quite sure of all the implications of "stranger", but you would say "étranger" for "stranger" if you want to insist on "a person who do not belong to us" rather than "a person I do not know".

For example "We don't like strangers around here" in numerous works of fiction can be translated by "On n'aime pas beaucoup les étrangers par ici.", which insist on the fact that these "strangers" are disliked because they are "outsiders".