[i18n] Language switcher for anon users

[rajatvijay]

Related #2927

Technical details

This PR does the following things -

1. Language switcher for the anon pages.
2. Save the selected language in the cookies to be preserved during re-loads.

Notice:

1. No matter what the user selects here, the preferred_language saved in the DB always takes the preference to translated pages once the user has logged in.
2. The default value of preferred_language user's model attribute is en so that attribute will always have a value. The ideal way would have been to give cookies lang preference for the users who are logging in for the first time. But this needs some discussion.
3. This logic can be found here but the language switch functionality for anon users is introduced here hence mentioned this product related point in this PR

Screenshots

Checklist

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• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[rajatvijay]

@ghislaineguerin @kgodey assigned you for a product related review.

[silentninja]

Looks good!

[silentninja]

Can you add comments where the set_language url coming from

[rajatvijay]

de03c76

[kgodey]

Looks fine to me.

[seancolsen]

This looks mostly good. I've requested one minor change. Also, we'll need a way to hide this UI if we merge this before the Japanese translation is complete.

[ghislaineguerin]

Looks good to me.

[rajatvijay]

Commented out the UI element to hide it until complete i18n work is merged. de03c76

[seancolsen]

Looks good now 👍🏻

[seancolsen]

I'm holding off merge since this is stacked on top of #3103. @rajatvijay feel free to merge this yourself once that PR is merged.