How do I write a story with foreign language components?

[Ally]

I am writing a novel. My English speaking characters are in a foreign country. They talk to other people from that country who probably are not readily speaking English, but their native language, let's say German. Would a reader be satisfied that the words of the foreign person were in English? Or do I need to add interpreters for validity? Or can I make some general statement in a preface to the reader. Do I need explanations about each person from that country and why they are suddenly speaking English? Thank you readers