Every language has accents, dialogs, and so on. Even sign language (and they still won't know not everything is American Sign Language. There's British, French, Japanese, etc.) has accents. I'm struggling to think of a language that doesn't. Maybe Basque? Really it's a question of geography and time. You drop a canyon in the middle of Rhode Island, wait 1,000 years, and the people on either side won't be able to understand one another.