Why are seemingly foreign words such as hors d’œuvres, maître d’, garçon, and Gesundheit used in American vernacular?
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All languages in the world have borrowed words from other languages. That is how language works. In the title to your question there are four words of non-English origin: "foreign", "used", "modern" and "vernacular".
There are words that are retained in their original language because they impart a certain feel, say, of elegance, snobbery, belonging, etc. hors d' oeuvres, maitre d', and garçon were retained by French restaurants, and came to be generalized because of the connotation of elegance and sophistication of French restaurants. The same could be said of concierge in a hotel.
There are words that perfectly capture a situation in a foreign language, which cannot be captured so precisely in English. An example of that would be schadenfreude (the pleasure derived from the misfortunes of others.) English 'borrows' them.
There are words that have been carried over from waves of immigrants, words that were used enough by a group, or captured the fancy of other English speaking people, that they stayed in the language. A typical example of this is the story of the Pennsylvania Dutch.
We inherited German words like Verboten, or Gott in Himmel (the im having been changed to in), and they have adopted English into their dialect: outen the lights is a common expression around the Lancaster area of Pennsylvania, spoken by non-Deutch people.
The reasons are varied, but there are several that are most prominent. I've listed three.
This is mostly anecdotal (I can't find/don't have/remember my sources):
EDIT: I found something in the way of a reference, but it doesn't corroborate exactly what I have here: http://www.uta.edu/english/tim/courses/4301w99/lge.html