Here's why:
* Visuals and Themes: Altea's design, including architecture, clothing, and the characters themselves, heavily draws inspiration from Ancient Greece.
* Marth's Name: His name, "Marth", is a slightly altered version of "Martius", the Roman god of war. This further strengthens the Greek connection.
* Lack of a Defined Language: The Super Smash Bros. games haven't explicitly mentioned an Altean language.
It's important to note that the developers have never officially confirmed the exact language Marth speaks. It's a creative interpretation based on the visual and cultural cues within the game.