Our whole site is English for a reason and that reason isn't that we don't like other languages. Our website is hosted in a primarily English-speaking country and all our moderators speak English. Some speak other languages, but every single one of us speaks English and that's the easiest language for us to moderate so we can keep the site flaming, profanity, and spam free.

In other areas of the site, other languages can be allowed via the person typing in their native/most comfortable language and then providing an English translation, but in chat that isn't as viable due to how long those messages get very quickly. Private chat rooms are a planned feature, according to Paril, so these would more easily accommodate users who speak the same language that isn't English in an area where they don't need to be as heavily monitored by mods who have to translate every single message just to do their job.

As for your actual suggestion, I don't think it's really necessary. As Pepijn said, most people I've encountered speaking other languages in chat either know English well and are purposefully speaking another language, leave the chat room when they're asked to speak English, or outright refuse to speak English when asked. I don't think a bot would really change any of those things and it wouldn't really be any more effective or convenient than a moderator doing the same thing.