title: concat string and variable in twig
body: <p>I have this in my template </p>
  
  <pre><code>    {{ ad.title_de }}
  </code></pre>
  
  <p>Now I'm incorporating multiple languages, so 'title_de' has to change
  I also have a variable 'tld' which  is one of  de, en , fr</p>
  
  <p>So I'd like to have something like </p>
  
  <pre><code>    {% if tld == 'fr' %} 
      {{ ad.title_fr }} 
  </code></pre>
  
  <p>etc
  Any ideas?</p>