Universal language

See also: translator

In case of everyone speaking English or similar, this should be considered only when you'd expect them to speak some other language. For example, your standard space aliens, they're unlikely to speak English or even translate their own speech fluently into English suddenly.

Sci-Fi instances tend to have universal translators, but even they occasionally fail at it. Such as Star Trek where plenty of people speak with differing languages (Wookiees with their howls being a very obvious example) yet understand each other perfectly. Somehow everyone conveniently understands the "universal" language used by the protagonists and they understand everything spoken to them by others.

In fantasy settings this is so common it's probably hardly worth noting. Though references to their language is clear sign of universal language at work.