Export: change translations tag

[derpixler]

at the current version of export files the translation relations will saved as a single tag with attributes.

<wp:translation lang="de" post_id="2485"/>
<wp:translation lang="nl" post_id="769"/>
<wp:translation lang="es" post_id="2563"/>
<wp:translation lang="fr" post_id="2594"/>
<wp:translation lang="it" post_id="2546"/>

For consistency parsing we want use this format.

<wp:translation>
    <wp:locale>nl_NL</wp:locale>
    <wp:element_id>4711</wp:element_id>
</wp:translation>