Part of it is who the intended audience is. You want the target audience to be able to understand what is going on. If the target audience typically only knows one language, you don't want them to be confused about what other characters are saying. So that means forcing characters to speak a language that may not technically fit the story, or adding subtitles.
Schubert125 t1_jeg4mwc wrote
Reply to Why do so few American movies let foreign language speaking characters speak their own language? Why does everything have to be in English... by _wyfern_
Part of it is who the intended audience is. You want the target audience to be able to understand what is going on. If the target audience typically only knows one language, you don't want them to be confused about what other characters are saying. So that means forcing characters to speak a language that may not technically fit the story, or adding subtitles.