"Will we eat together tonight?"
Translation:On mange ensemble ce soir ?
On is special in that even though it is informal we , its conjugation follows the third person singular form of il/elle https://www.frenchtoday.com/blog/french-grammar/understanding-french-on-subject-pronoun/#:~:text=The%20French%20pronoun%20%E2%80%9COn%E2%80%9D%20means,%2C%20she%2C%20and%20even%20I!
On is a form of the third person singluar which is commonly used to mean 'we', it retains its corresponding verb form regardless of its use in a sentence. It's not plural per se but it is often used that way. Mangent (ils/elles) would be the third person plural (they) and wouldn't fit in a 'we' context.