"Maria is a foreigner."
That's a fair call, but including or not including さん is considerably more complicated than "being/not being rude".
Contrary to what @StevenPaul5 said, マリアは外国人です is not informal, and not necessarily rude. Formality and politeness are not always one and the same in Japanese.
Here, です is used, so the speaker is being "formal" (basic civilized society level formal; there are other higher levels of formality in Japanese) towards the listener. But by choosing not to use さん with Maria, the speaker indicates that they are either close friends with her or they have been explicitly told by Maria that she doesn't need to be addressed with さん and are respecting Maria's foreign-ness.
Further to this, the most likely explanation (setting aside any foreign complications) is the 内・外 うち・そと concept, which is central to Japanese society and essential to understanding social nuances.
To elaborate a little more, of the face of it, we should assume that Maria is part of the speaker's in-group (内), and that the listener is not.
Love your comment about the formality and politeness distinction - I'd add the degree of familiarity (or friendliness) with the person as another kind of distinction making politeness, formality, and familiarity as a kind of tripartite consideration set for addressing individuals. I lived in Japan for a year, like probably many on this site, and learned not only how important these choices (often made unconsciously) are, but how confusing they can be for learners. For example, I saw colleagues in Japan often addressing each other with ちゃん (chan) or "kun"- they were being informal but not impolite since familiarity was high. I couldn't do it because I hadn't known them long enough, so I would have been rude. These kinds of issues are true for all languages I think - perhaps hardest to detect in our native tongues, since we're often just semi-conscious of our choices. This site does not a bad job with the san, kun, chan distinction https://www.thoughtco.com/how-to-use-san-kun-chan-4058115
Adding a step further on making the distinction of polite and formal, what style is the Japanese Wikipedia written in?
It uses である instead of です or だ, ではない instead of じゃない or ではありません, and other plain forms for verbs and adjectives. Some sources say that is a formal style, but not a polite one.
JoshuaLore9's comment did make a distinction of polite and formal but called です formal, unlike other sources that called it polite. This has confused me, is there a way to distinguish polite and formal in either English and Japanese?
が can be used in this sentence, but it's not very common/natural in most situations I can think of. The difference between は and が is very difficult to explain, mostly because it comes down to the interplay between context and intention so it's hard to make consistent rules for. (Note, this is only for when you have to choose between the two, not when both are present in the sentence which is tricky too because which noun you assign to which particle depends on intended emphasis as well.)
The only scenario I could come up with where が would sound natural in is if you asked someone "you know that foreigner?" and they said "oh, you mean Ken?" to which you replied "no, Ken is short for Kentarou who is Japanese. Maria is the foreigner. (マリアが外国人です)"
So I would say マリア
は外国人です is more like "Maria is a foreigner", whereas マリア
が外国人です would be "Maria is a foreigner".
Regardless of all the above comments, in a previous question Duolingo marked me wrong for omitting さん、insisting that it would ONLY accept it with さん。 This is the absolute worst kind of sin for an alleged educator! To randomly reverse which is "correct". Without any context given, it's just a guess. Shame Duolingo, shame.!