Add ops flag to disable cookie-based language switcher

What does this MR do and why?

To resolve #395008 (closed). More context information here: #395008 (comment 1705529958)

Add ops flag to disable cookie-based language switcher.

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Screenshots or screen recordings

Before	After

How to set up and validate locally

• Run Feature.enable(:disable_preferred_language_cookie) in your GDK's console.
• Sign out and check the bottom right of the sign-in screen.
• See also http://gdk.test:3000/users/sign_up and http://gdk.test:3000/-/trial_registrations/new.