Revert "Don't symbolize tainted data." [ci skip]

Reason: i18n whitelists now locales without passing through symbols,
see https://github.com/svenfuchs/i18n/blob/master/lib/i18n.rb#L278.
Therefore, this snippet is no longer a good practice.

This reverts commit ec0664a6.

guides/source/i18n.md

@@ -145,11 +145,7 @@ The _setting part_ is easy. You can set the locale in a `before_action` in the `
 before_action :set_locale
 
 def set_locale
-  if %w[en fr].include?(params[:locale])
-    I18n.locale = params[:locale]
-  else
-    I18n.locale = I18n.default_locale
-  end
+  I18n.locale = params[:locale] || I18n.default_locale
 end
 ```