Hello,

As you've noted correctly the content is set in the XML file at
..surveywap/xmldata/languages/....

en.xml or en-us.xml (and other language files)

Entry: 
  <data version="1.0" id="143" name="InvitationMessageTextBox">
    <value>
      Dear Sir, Madam,
      &lt;br /&gt;&lt;br /&gt;
      We kindly invite you to participate in the "{0}" survey.
      &lt;br /&gt;&lt;br /&gt;
      Please open the following link in your webbrowser: &lt;br /&gt;
      &lt;a href="{1}"&gt;{1}&lt;/a&gt;
      &lt;br /&gt;&lt;br /&gt;
      Or enter the following personal code if needed:
      [--invitationid-]
      &lt;br /&gt;&lt;br /&gt;
      Kind regards,&lt;br /&gt;
      {2}
    </value>
  </data>

If changes work locally they should work on a third party server. Possibly empty the caching or refreshing the live site may help.

Also make sure your browser language settings are adjusted to the changed language files (en.xml or en-us.xml) and the serversettings.