I'm sorry, but the knowledge sources do not contain any information about Mixpanel automatically collecting or providing a default user property for "preferred language" (such as a property like $preferred_language or similar). The documentation lists default user properties that are automatically collected—such as city, region, country, timezone, OS, browser, etc.—but "preferred language" is not mentioned among them.
If you want to track a user's preferred language, you would need to explicitly set this as a custom user profile property using your implementation (for example, by calling .people.set() in the SDK after identifying the user and passing the preferred language value) Docs: Storing User Profiles (JavaScript).
If you need to collect this property, you should capture the user's language preference in your application and send it to Mixpanel as a custom property on the user profile.
