How to migrate to translatable fields
I solved the problem by first doing the field translation manually for one node, and then using Node Export to see the actual code describing the node.
Finally, I used the prepare function of the Migrate api to do the work manually.

function prepare($entity, stdClass $row){

    $entity-&gt;language = 'el';       
    $entity-&gt;title_field['el'][0]['value'] = $row-&gt;name;
    $entity-&gt;title_field['en'][0]['value'] = $row-&gt;name_en;
    $entity-&gt;translations = (object) array(
                    'original' =&gt; 'el',
                    'data' =&gt; array(
                            'el' =&gt; array(
                                    'entity_type' =&gt; 'node',
                                    'entity_id' =&gt; $entity-&gt;nid,
                                    'language' =&gt; 'el',
                                    'source' =&gt; '',
                                    'uid' =&gt; '0',
                                    'status' =&gt; '1',
                                    'translate' =&gt; '0',

                            ),
                            'en' =&gt; array(
                                    'entity_type' =&gt; 'node',
                                    'entity_id' =&gt; $entity-&gt;nid,
                                    'language' =&gt; 'en',
                                    'source' =&gt; 'el',
                                    'uid' =&gt; '1',
                                    'status' =&gt; '1',
                                    'translate' =&gt; '0',

                            ),
                    )
            );
 }