The fear of strangers or foreign people is often known officially as Xenophobia. It can not only be a fear of people they have not met, but of foreigners to their country and people with different beliefs. Xeno means stranger and phobia means to have a fear.
The fear of foreigners and people different to ones self can be regarding anybody from a different culture to your own, people with different personalities, higher levels of confidence or just somebody who believes in different things to yourself. People speaking in a foreign language can also cause alarm for those suffering with this phobia or fear and will always give a cause for alarm.
Xenophobia is something not decided on, but more like a natural instinct to the person holding the fear. Suffering feeling these emotions strongly feel genuinely that people different to themselves in any shape or form could be something to be wary of, so they usually either confront or back well away from the potential danger.
There are many Xenophobics who suffer in such an extreme way that it causes them to become nasty and violent towards others. They can lash out and say rude and racist remarks. The majority of sufferers, however will stay calm and deal with it away from others in a safe place.
Throughout history and even today, tribes across the world follow some kind of Xenophobia and will fight any tribes different to themselves. This just shows how many different levels it comes it and how widespread it can be. Hitler was the most extreme active Xenophobic in his day and due to his extreme feelings, had millions of Jews gassed and killed. This was all because they were different to him so he had his soldiers do the dirty deed of destroying those that he took a strong dislike to.