I like this pattern that Wordpress uses on their login page.
Why it's a good pattern:
- The user encounters it as early in the workflow as possible (login)
- The icon has both a Roman and a Mandarin character
- Changing the value instantly changes everything on the page, so the user sees that it's a language selector
If your system doesn't require a frequent login (like an app that saves login tokens for six months), I'd put something like this on the user's profile page as close to their name as possible. Everyone clicks on their name or face to find their personal preferences... this avoids making them wade through system-level settings, which might be harder to navigate in another language.