Provide support to fix locale selection in admin login page

[gsmendoza]

Expected behavior

Given I am running a Solidus app with Solidus I18n (see https://github.com/gsmendoza/rails_7_store/tree/solidus_30253731078c_3_2_0_alpha_solidus_i18n)

And I am in the admin login page

When I change the locale in the locale selector (e.g. to Espanol MX)

Then I should see the locale change on the login page

Actual behavior

The "Loading" popup window appears but nothing happens.

In the backend, a PUT request is submitted to "/admin/locale/set" but the request is redirected to the login page, e.g.

Started PUT "/admin/locale/set" for ::1 at 2022-08-04 17:24:49 +0800
Processing by Spree::Admin::LocaleController#set as JSON
Parameters: {"switch_to_locale"=>"es-MX"}
Redirected to https://localhost:3000/admin/login
Completed 200 OK in 11ms (ActiveRecord: 0.0ms | Allocations: 4703)

If you try to log in afterwards, you are redirected to https://localhost:3000/admin/locale/set with the following error:

Routing Error: No route matches [GET] "/admin/locale/set"

Causes

1. Non-logged in users are not authorized to change the admin locale.
2. When the language is changed on the admin, SolidusAuthDevise's admin controllers do not react to this, because their set_user_language_locale_key is not set to :admin_locale.

Background

solidusio/solidus_auth_devise#224 was submitted to fix the second cause of the bug: set_user_language_locale_key not being set to :admin_locale. Based on the discussions in the PR, we decided to update Solidus in two ways:

1. Fix the first cause of the bug: Non-logged in users are not authorized to change the admin locale.
2. Extract SetsUserLanguageLocaleKey so that Fix locale inconsistence and remove redundant template solidus_auth_devise#224 won't have to refer to :admin_locale key directly.

Fix Demo

Tested manually with this Rails app: https://github.com/gsmendoza/rails_7_store/tree/gsmendoza/eng-436-changing-the-locale-in-the-admin-login.

The Rails app includes this corresponding fix for SolidusAuthDevise: https://github.com/gsmendoza/solidus_auth_devise/tree/gsmendoza/eng-436-changing-the-locale-in-the-admin-login.

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[waiting-for-dev]

Thanks, @gsmendoza!

[kennyadsl]

Thanks @gsmendoza!