In MD Settings, tapping a language[1] in the language list on the Advanced page immediately sets that language as the user's UI language (Windows/CrOS), which will take effect on the next Chrome restart.

Changing the UI language probably (I'm guessing) happens at most once in the life of the majority of devices, so it doesn't have to be easy. Accidents can be quite difficult to recover from, as even if you have passing knowledge of a language you may not be able to navigate a technical settings page in that language. And the change takes effect after the next restart, so the user won't associate the language "changing itself" with their fiddling in Settings.

There should already be a toggle button on the language details page (which may need some UI TLC). If we need something top-level, we should make it harder to do accidentally.

[1] or the dropdown menu next to a language, but that's a bug:  issue 636223