The answer is a little tricky, because the show's creators have never officially established whether the residents of Springfield speak a specific language other than English.
Here's what we know:
* The show is primarily in English. The main characters all speak English, and the majority of the dialogue is in English.
* There are some instances of other languages. There are a few episodes where characters speak other languages, usually for comedic effect. For example, in the episode "The Last Temptation of Krusty", Krusty the Clown speaks French. In "The Italian Bob," a whole storyline is about Italian heritage. But these are just a few examples, and they don't suggest that the entire town speaks multiple languages.
* The show often satirizes American culture. The creators probably intentionally leave the language of Springfield ambiguous, focusing on the humor of American life.
Ultimately, it's up to you to decide. You can choose to believe that the citizens of Springfield primarily speak English, or you can imagine them speaking other languages, just for fun. It's part of the beauty of a fictional world!