In your user settings, make sure “Language” is set to both “English” and “Undetermined”. You can select additional languages as well, but they aren’t currently allowed on this server. On desktop, you can use CTRL+click to select both options. This will allow you to see content for which the author has not set a language, and content which has been set to “English”.
That has not been my experience. I did see a post in French once, but that's it. Can only assume non-English speakers are way better at using their language settings than we are.
In theory, you shouldn't have to use the language box at all when creating a post or comment - if you don't touch it, the system is supposed to auto-assign it.
Sadly, I don't know if this is on the devs' radar for clarification.