Migrating translations using drupal migrate.

translation.migrate.php

<?php

/**
 * Migration class for migrating the source translations.
 */
class MySourceTranslationMigration extends Migration {
  // Nothing special here. Just set up a node migration, migrating to nodes
  // that will serve as translation sources.
  // ...

  public function __construct() {
    // Set up everything for your migration.
    // ...

    // An example map.
    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'my_id' => array(
          // ...
        ),
      ),
      MigrateDestinationNode::getKeySchema()
    );
  }
}

/**
 * Migration class for migrating the translations.
 */
class MyTranslationMigration extends Migration {
  public function __construct() {
    // Set up everything for your migration.
    // ...

    // Make sure the source translations are a dependency.
    $this->dependencies = array('MySourceTranslation');

    // Set up the mapping for the tnid. The second argument must be equal to
    // the mapping key of the source translation migration.
    // The sourceMigration method call ensures that we fetch the nid from the
    // MySourceTranslation migration.
    $this->addFieldMapping('tnid', 'my_id')->sourceMigration('MySourceTranslation');
    
    // ...
  }

  /**
   * Implements prepare().
   *
   * If we're translating a node we need to make sure the translation set is
   * correctly initialized. This means the source translation needs its tnid set
   * to its own nid. Drupal does this on insert when a node has a its
   * translation source node assigned to a translation_source property.
   */
  function prepare(&$node, $row) {
    if (isset($node->tnid) && ($source = node_load($node->tnid))) {
      $node->translation_source = $source;
    }
  }
}