Perform a check for non-standard site languages to reduce "Incorrect locale information provided" errors

[jamesozzie]

Bug Description

If a user has an invalid site language defined, which would be through a third party plugin or as a WP constant, the Site Kit dashboard may not be accessible, with errors appearing on the screen, and more via console errors.

To resolve this, perform a check for any invalid language before set up or upon visiting the dashboard. Output an error or some context if possible.

The below is on a custom hosted site with an invalid language set via wp-config

The below screenshot is on a TasteWP site with an invalid language set via :

add_action('locale', function(){
   return '12345_INVALID';
});

Steps to reproduce

1. Set up Site Kit
2. Define an invalid language via wp-config or functions.php (see above example in screenshot)
3. Check the Site Health status to ensure the invalid language is applied
4. Visit the Site Kit dashboard, where the error appears

Additional Context

• Site Kit 1.109.9
• This is rare, but it did come up twice in the forums
• Internal testing

Impacted users

• https://wordpress.org/support/topic/problem-with-sitekit-dashboard/ | Open - awaiting feedback
• https://wordpress.org/support/topic/incorrect-locale-information-provided-need-suport/ | Resolved - no response | Site Language

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