title: prefix sylius product show url with locale
body: <p>I am using sylius v0.18. I want to prefix locale with product show urls.</p>
  
  <pre><code>sylius_core:
  routing:
      %sylius.model.product.class%:
          field: slug
          prefix: /p
          defaults:
              controller: sylius.controller.product:detailsAction
              repository: sylius.repository.product
              sylius:
                  template: SyliusWebBundle:Frontend/Product:show.html.twig
                  criteria: {slug: $slug}
                  permission: false
  </code></pre>
  
  <p>I can use a static word as the prefix in this configuration. but It does not work with _locale. <code>prefix: /{_locale}/p</code></p>