#7465013 Update loading status for A/B test on the domains step

This is a follow up proposal for #66246 after reviewing the experiment related code.

Proposed Changes

• Render a placeholder (or nothing for the second subtitle) while the experiment is loading
• Stop loading the experiment and render always ‘control’ experience on a non-EN page. UX-wise, this will also prevent the users of other languages to see the loading placeholder

Testing Instructions

Same as #66246 with these notes

• For EN locale, a placeholder will be displayed for subtitle 1 and CTA link while the experiment is loaded
• Even if a user is assigned to ‘treatment’, they will experience ‘control’ if the page is the locale is different than EN

Demos

• Treatment, before this change

• Treatment, after this change

• Control, for EN locale

Pre-merge Checklist

• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?