First try, should work.

includes/datasource_translated_entity.inc

@@ -0,0 +1,35 @@
+<?php
+
+class EntityTranslationDataSourceController extends SearchApiEntityDataSourceController {
+
+  /**
+   * Get a metadata wrapper for the item type of this data source controller.
+   *
+   * @param $item
+   *   Unless NULL, an item of the item type for this controller to be wrapped.
+   * @param array $info
+   *   Optionally, additional information that should be used for creating the
+   *   wrapper. Uses the same format as entity_metadata_wrapper().
+   *
+   * @return EntityMetadataWrapper
+   *   A wrapper for the item type of this data source controller, according to
+   *   the info array, and optionally loaded with the given data.
+   *
+   * @see entity_metadata_wrapper()
+   */
+  public function getMetadataWrapper($item = NULL, array $info = array()) {
+    if (is_array($info['property info alter'])) {
+      $index = reset($info['property info alter']);
+      $target_language = $index->options['entity_translation_language'];
+      if ($target_language != ENTITY_TRANSLATION_SEARCH_API_NO_TRANSLATION) {
+        // This may not be really needed.
+        $info['langcode'] = $target_language;
+        $wrapper = parent::getMetadataWrapper($item, $info);
+        $wrapper->language($target_language);
+        return $wrapper;
+      }
+    }
+    return parent::getMetadataWrapper($item, $info);
+  }
+
+}