Querying XML with Unicode data
Add the N prefix to the INSERT statement to preserve the XML as Unicode, eg

    insert into #test (Document) values
(N'&lt;FormDocument xmlns="http://mynamespace.org/Schemas"&gt;
  &lt;Fields&gt;
    &lt;Field Id="a" Value="qwerty" /&gt;
    &lt;Field Id="ą" Value="123" /&gt;
  &lt;/Fields&gt;
&lt;/FormDocument&gt;')