If something is foreign, it doesn’t belong where it is.
A foreign body is an object that shouldn’t be in your body. A foreign person (or foreigner) is someone who shouldn’t be in the country she’s in because she grew up somewhere else.
Applied to people, “foreign” is a lot like “them.” It doesn’t mean much unless you know who said it. Americans are foreign in China, Swedes are foreign in Germany, no one is foreign in their home country, and so on.
But also like “them,” “foreign” can make you feel like you’re in a special club, that you belong where you are and certain other people don’t belong there because they’re not like you.
Be careful with words that do this.