This PR improves the language selection.
Previously, we would miss the user's languages that included a locale.
For example, if a user's languages were `['en-US', 'fr'], then they
would get 'fr' because 'en-US' wasn't in our table—though 'en' was!
We were already doing the splitting elsewhere but now we do it here,
too.
### Release Note
- Improves default language
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Co-authored-by: Lu[ke] Wilson <l2wilson94@gmail.com>
5ab93eef5f