Is there a term for the "treatment of foreigners as outsiders"?

Of xenophobia, "While it may represent a true fear, most xenophobic people do not have a true phobia. Instead, the term is most often used to describe people who discriminate against foreigners and immigrants."

[verywellmind.com]

Of second-class citizen, "A person, belonging to a social or political group, whose rights and opportunities are inferior to those of the dominant group in a society.

[Lexico]

Perhaps 'ostracism'? OED Lexico defines it as:

Exclusion from a society or group.

It can have meanings pertinent to the specific aspect of foreigner relations such as Linguistic ostracism while also having less targeted animosity present in words such as 'aversion'.

You could use prejudiced. - meaning - "having or showing a dislike or distrust that is derived from prejudice; bigoted"

Possibly bigotry- "obstinate or unreasonable attachment to a belief, opinion, or faction; in particular, prejudice against a person or people on the basis of their membership of a particular group."

Although, as @Rem stated, the literal definition of xenophobia is simply a fear and hatred of strangers or foreigners or of anything that is strange or foreign, it often carries with it a negative connotation which is why you probably wish to avoid it. It carries with it a similar weight to "racism" or "bigotry".

The closest that you can get to the idea which you are describing is probably simply going to be "treated like a foreigner", or "treated like a nonnative". That's the general phrase that is used when referring to such a situation. Although you can use words such as "ostracization" and "exclude" those are really only for certain cases and are not representative of the general situation that you are referring to, in addition to a general negative connotation brought forth. There isn't a specific term that describes this scenario.

other and otherism, often other is capitalized: Other

And it can apply to foreigners, also.

treating a person as the other

Here is one description of it:

Otherism, in its tendency to dehumanize the “others”, has pervaded our culture, and perhaps has never been so discussed as in recent days. Mentions of racism, white supremacy, sexism, and classism wallpaper our national and local news.

otherism

In ART:

This investigation has led me to explore the commonalities and differences in the visual representation of racially constructed Others more broadly, and I keep reaching the same conclusion: no visual representation of people of color is ever innocent or powerful enough to challenge the socially defined images that frame them—depending on the given illusion—as marginal, as threats, as foreigners, as Others, as having too much culture or too little culture. On their own, artistic representations can do little to challenge racism. It takes structural change to create a visual revolution that can fully change and destroy our illusions.

Others and art

In FILM: City Archives (1977) 28 mins| Short

Foreman produced City Archives, a labyrinthine collage of image and language, at the invitation of the Walker Art Center in Minneapolis. The tape centers on the perspective of an outsider — the foreigner as Other — towards a city and its artifacts. Foreman’s signature visual and verbal puns and carefully composed compositions result in an often humorous dialogue on the role of documents as evidence, and the relationship of text and image. While questioning the positions from which one views information, Foreman constructs and then deconstructs the central metaphor of an archive as a receptacle of information and knowledge. Richard Foreman, filmmaker

One of the first to theorize about this was Julia Kristeva in her many works such as the book Black Sun. Ultimately, we project otherness onto others because we cannot handle those parts of ourselves that are other to us.

Julia Kristeva and otherism

The concept of the other in, among others,:

Philosophy, psychology, ethics, critical theory, etc. [NO psychiatry]

The Other, philosophy and psychoanalysis

These foreigners are Gaijin. a Japanese word for foreigners or non-Japanese nationals. The use is often neutral, although it may also be seen as derogatory.

gaijin: a foreigner (in Japan) Examples of gaijin in a Sentence - Recent Examples on the Web

Some gaijin, like Kelly Luce and Pico Iyer, had been touched by Japanese culture deeply enough to write about it. — Aaron Gilbreath, Longreads, "Japan: A Longform Reading List of Longform Writing," 30 Apr. 2020

Through conversations with Japanese friends and other gaijin, Iyer illuminates the uniquely paradoxical culture that defines present-day Japan: a society alone, together; at once preposterous and repressed; equally ordered and chaotic. — Ashlea Halpern, Condé Nast Traveler, "Best Books to Read if You're Dreaming of Japan," 30 Mar. 2020

Merriam webster

I do not repeat here the relevant supporting material to be found in other dictionaries or in https://en.wikipedia.org/wiki/Gaijin

Perhaps you could use isolationism.

This is the definition:

i·so·la·tion·ism noun: isolationism a policy of remaining apart from the affairs or interests of other groups, especially the political affairs of other countries. "the country chose a policy of isolationism that made it a secondary player in world political events"

Isolationism is different from racism or xenophobia because it is saying the people tend to keep to themselves more, not having anything to do with hate.

Although it is often used in politics and international relations, it can also be used as you said in your question: that some specific cultures may be isolationist because they have been that way for hundreds of years.

If you are looking for a specific term for being an outsider because you are from a different country as defined by artificial borders, as opposed to a different culture, I can't think of a good one. I would stick with "outsider" as a general term for this in relation to cultural newcomers, and am particularly drawn to this part of the second definition - "A person...who feels different from those people who are accepted as members". It seems to encompass what you are describing - not from here - not familiar with our ways - not part of our shared experience. Even in the most welcoming cultures, newcomers retain these traits for a long time.

Many other terms are specific to a particular culture, and if you are looking for a particular one, you can peruse this list.