title: Silverstripe TreeDropDownField is showing (none) in translation
body: <p>I am using SilverStripe 3.1.2.
  A page has a $has_one to SiteTree and the TreeDropdownField is implemented like this:</p>
  
  <pre><code>$firstOfferBoxLink = new TreeDropdownField(
      'FirstOfferBoxLinkID', _t('General.FIRST_OFFER_BOX_LINK', 'First offer box link'), 'SiteTree'
  );
  $fields-&gt;insertBefore($firstOfferBoxLink, 'FirstOfferBoxImage');
  </code></pre>
  
  <p>In the default language this is working like it should, but in translations of the page it just shows:</p>
  
  <p>"Link    (none)"
  I have tried following the instructions on the github repo (found <a href="https://github.com/silverstripe/silverstripe-translatable/blob/master/docs/en/index.md" rel="nofollow">here</a>), for example with the following line after all the normal fields but before the TreeDropDownFields:</p>
  
  <pre><code>$this-&gt;applyTranslatableFieldsUpdate($fields, 'updateCMSFields');
  </code></pre>
  
  <p>Nothing changes.</p>
  
  <p>Has someone got any idea, how to get TreeDropDownFields to work in translations?</p>