Fix inconsistence between the locale on the backend and the locale in…

… the devise's backend controllers

NOTE: ::Spree::Admin::SetsUserLanguageLocaleKey concern added in Solidus 3.2, so as soon as this gem stops supporting Solidus versions < 3.2 this "if" should be replaced with simple include

lib/controllers/backend/spree/admin/user_passwords_controller.rb

@@ -42,4 +42,15 @@ def update
       super
     end
   end
+
+  private
+
+  # NOTE: as soon as this gem stops supporting Solidus 3.1 if-else should be removed and left only include
+  if defined?(::Spree::Admin::SetsUserLanguageLocaleKey)
+    include ::Spree::Admin::SetsUserLanguageLocaleKey
+  else
+    def set_user_language_locale_key
+      :admin_locale
+    end
+  end
 end

lib/controllers/backend/spree/admin/user_sessions_controller.rb

@@ -35,6 +35,15 @@ def authorization_failure
 
   private
 
+  # NOTE: as soon as this gem stops supporting Solidus 3.1 if-else should be removed and left only include
+  if defined?(::Spree::Admin::SetsUserLanguageLocaleKey)
+    include ::Spree::Admin::SetsUserLanguageLocaleKey
+  else
+    def set_user_language_locale_key
+      :admin_locale
+    end
+  end
+
   def accurate_title
     I18n.t('spree.login')
   end