title: Fallback method for translation
body: <p>Is it possible to have a fallback method for an translation that's not found in the messages catalog?</p>
  
  <p>I have <code>"wave.transition.delete_tasks.display"|trans</code> and instead of returning that string, I want to return "Delete Tasks".</p>
  
  <p>I could run a cleanup after every call, e.g. </p>
  
  <pre><code>"wave.transition.delete_tasks.display"|trans|cleanup
  </code></pre>
  
  <p>where <code>cleanup</code> does exactly that, but I was hoping for a hook where I could always run it.</p>
  
  <p>My current plan is to combine <code>|trans|cleanup</code> into <code>|clean_trans</code>, and running the translation lookup in the Twig filter, but would prefer an easier way.</p>