Imagine you're on another planet. Its inhabitants don't understand you, but you understand them perfectly. You do not have Translator Microbes in your ear; you really are hearing the alien language and understanding it perfectly.
Simply put, this is a character who is able to understand aliens. If the alien sounds are not physically impossible for a human mouth to reproduce, expect the character to speak their language, too. If the aliens can communicate with all humans, it is likely that the aliens are just speaking English. If the aliens are only shown to be able to communicate with one human, they are meant to be speaking their own alien tongue that the human happens to be fluent in.
Please note that this is when a character inexplicably understands an alien language. If a human character can speak an alien language because they learned how to, it is no different than a native English speaker learning a foreign language and is not a trope.
Compare Speaks Fluent Animal, for when a character is able to understand animals. Usually, but not always, overlaps with the Translation Convention so the viewer can follow the conversation. May overlap with Suddenly Fluent in Gibberish if the ability springs out of nowhere. Subtrope of Inexplicable Language Fluency.
Examples:
- Starlight: McQueen is able to communicate with aliens.
- Space Chimps: The titular characters and the aliens are able to communicate with each other. While it's possible that chimps and aliens just have a Common Tongue, in the sequel it's explicitly shown that Zartog speaks some strange alien language that doesn't resemble chimp noises at all.
- The Neverending Story: When Bastian enters Fantastica, he has no problem conversing with anyone.
- Roark: Mira Donazetto works as a translator for the Godan.
- Phineas and Ferb: Ferb is shown to be able to communicate with aliens in one episode.
