title: Symfony 4 translations component translate by id not working
body: <p>When I call function <code>$translator-&gt;trans($key)</code>, the doc states that the first parameter is the id. To me it seems that the id should relate to the id attribute inside the .xlf file. In reality, it relates to the source element of the .xlf file.</p>
  
  <p>Method call inside controller:</p>
  
  <pre><code>$translator-&gt;trans('KUNGFU.CODER')
  </code></pre>
  
  <p>messages.en.xlf (EN) which does NOT work with the method call above:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;xliff version="1.2" xmlns="urn:oasis:names:tc:xliff:document:1.2"&gt;
      &lt;file source-language="nl" datatype="plaintext" original="file.ext"&gt;
          &lt;body&gt;
              &lt;trans-unit id="KUNGFU.CODER"&gt;
                  &lt;source&gt;De code fu is sterk bij deze&lt;/source&gt; // dutch source message
                  &lt;target&gt;The code fu is strong inside this one&lt;/target&gt;
              &lt;/trans-unit&gt;
          &lt;/body&gt;
      &lt;/file&gt;
  &lt;/xliff&gt;
  </code></pre>
  
  <p>messages.en.xlf (EN) which does work with the method call above:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;xliff version="1.2" xmlns="urn:oasis:names:tc:xliff:document:1.2"&gt;
      &lt;file source-language="nl" datatype="plaintext" original="file.ext"&gt;
          &lt;body&gt;
              &lt;trans-unit id="KUNGFU.CODER"&gt;
                  &lt;source&gt;KUNGFU.CODER&lt;/source&gt; // id
                  &lt;target&gt;The code fu is strong inside this one&lt;/target&gt;
              &lt;/trans-unit&gt;
          &lt;/body&gt;
      &lt;/file&gt;
  &lt;/xliff&gt;
  </code></pre>
  
  <p>I could of course go ahead and use the second option everywhere but it seems wrong. What am I missing here?</p>