diff --git a/includes/common.inc b/includes/common.inc
index bce818d..7fe20b4 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -7796,7 +7796,14 @@ function entity_language($entity_type, $entity) {
   }
   else {
     // @todo The right value for D8 should be LANGUAGE_NONE, we cannot use it
-    // here to preserve backward compatibility.
+    // here to preserve backward compatibility. In fact this function has been
+    // introduced very late in the D7 life cycle as the proper default value for
+    // field_attach_form(). By returning LANGUAGE_NONE when no language
+    // information is available, we would introduce a potentially BC-breaking
+    // API change, since field_attach_form() defaults to the default language
+    // instead of LANGUAGE_NONE. Moreover this allows us to distinguish between
+    // entities that have no language specified from ones that do not have
+    // language support at all.
     $langcode = NULL;
   }
 
diff --git a/modules/system/system.api.php b/modules/system/system.api.php
index c551e22..d749250 100644
--- a/modules/system/system.api.php
+++ b/modules/system/system.api.php
@@ -97,6 +97,14 @@ function hook_hook_info_alter(&$hooks) {
  *     instead specify a callback function here, which will be called to
  *     determine the entity label. See also the entity_label() function, which
  *     implements this logic.
+ *   - language callback: (optional) A function taking an entity and an entity
+ *     type as arguments and returning the language the entity has been created
+ *     in. This value may be changed when editing the entity and represents the
+ *     language its textual components are supposed to have. In most situations,
+ *     when needing to determine this value, inspecting a property named after
+ *     the 'language' element of the 'entity keys' should be enough. However
+ *     some entity types might not define such a property, in which case the
+ *     language callback can be used to return the proper value.
  *   - fieldable: Set to TRUE if you want your entity type to accept fields
  *     being attached to it.
  *   - translation: An associative array of modules registered as field
@@ -123,6 +131,9 @@ function hook_hook_info_alter(&$hooks) {
  *       'subject' should be specified here. If complex logic is required to
  *       build the label, a 'label callback' should be defined instead (see
  *       the 'label callback' section above for details).
+ *     - language: The name of the property that contains the entity language
+ *       code, typically 'language'. If no language property is available, the
+ *       'language callback' may be used instead.
  *   - bundle keys: An array describing how the Field API can extract the
  *     information it needs from the bundle objects for this type. This entry
  *     is required if the 'path' provided in the 'bundles'/'admin' section