First try, should work.

README.txt

@@ -0,0 +1,8 @@
+This module differs from this one: http://drupal.org/project/search_api_et. It 
+uses an alternative way of getting language aware search with entity translation: 
+have one index per language. This has some drawbacks like scalability and site 
+building usability. But when used with Solr as indexing backend, it allows to set 
+a different configuration for each language (by declaring one Search API server 
+per language, matching to a different Solr core). This especially makes sense if 
+the website only has two or three languages.
+For more info on these considerations, see http://drupal.org/node/1393058.

entity_translation_search_api.info

@@ -1 +1,6 @@
 name = Entity translation search API
+description = Allows to set several monolingual search indexes.
+package = Search
+core = 7.x
+
+files[] = includes/datasource_translated_entity.inc

entity_translation_search_api.module

@@ -0,0 +1,61 @@
+<?php
+
+define('ENTITY_TRANSLATION_SEARCH_API_NO_TRANSLATION', 'none');
+
+/**
+ * Implements hook_search_api_search_api_item_type_info_alter().
+ */
+function entity_translation_search_api_search_api_item_type_info_alter(array &$infos) {
+  foreach ($infos as $type => $info) {
+    if (entity_get_info($type)) {
+      $infos[$type]['datasource controller'] = 'EntityTranslationDataSourceController';
+    }
+  }
+}
+
+/**
+ * Implements hook_form_alter().
+ */
+function entity_translation_search_api_form_alter(&$form, &$form_state, $form_id) {
+  if (in_array($form_id, array(
+    'search_api_admin_add_index',
+    'search_api_admin_index_edit',
+  ))) {
+    if ($form_id == 'search_api_admin_index_edit') {
+      $index = $form_state['index'];
+      $default = isset($index->options['entity_translation_language']) ? $index->options['entity_translation_language'] : ENTITY_TRANSLATION_SEARCH_API_NO_TRANSLATION;
+    }
+    else {
+      $default = ENTITY_TRANSLATION_SEARCH_API_NO_TRANSLATION;
+    }
+    $languages = language_list('enabled');
+    $languages = $languages[1];
+    $names = array(ENTITY_TRANSLATION_SEARCH_API_NO_TRANSLATION => t('No translation'));
+    foreach ($languages as $langcode => $item) {
+      $name = t($item->name);
+      $names[$langcode] = $name . ($item->native != $name ? ' (' . $item->native . ')' : '');
+    }
+    $form['options']['entity_translation_language'] = array(
+      '#type' => (count($names) <= 5 ? 'radios' : 'select'),
+      '#title' => t('Entity translation language'),
+      '#description' => t('To which language the entities should be translated before indexing.'),
+      '#default_value' => $default,
+      '#options' => $names,
+    );
+  }
+}
+
+/**
+ * Implements hook_search_api_index_items_alter().
+ * For compatibility with language control data alteration.
+ */
+function entity_translation_search_api_search_api_index_items_alter(array &$items, SearchApiIndex $index) {
+  $target_language = $index->options['entity_translation_language'];
+  if ($target_language != ENTITY_TRANSLATION_SEARCH_API_NO_TRANSLATION) {
+    foreach ($items as $id => $item) {
+      if (isset($item->translations->data[$target_language])) {
+        $item->language = $target_language;
+      }
+    }
+  }
+}

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);
+  }
+
+}