If they’re even remotely like us, then they’ll have the same sort of story, which will entail multiple languages.

That is: To assume that a sapient, language-capable genus evolves in one place, then spreads across a planet from there.

Even if they all speak the same language when they begin the spread, the same sorts of factors will encourage semantic drift in different ways in different places, and over enough time, multiple languages are all-but inevitable.