diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index 44b80c872bb..1d5b39ae826 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -6685,28 +6685,18 @@ class core_string_manager implements string_manager {
     }
 
     /**
-     * Returns dependencies of current language, en is not included.
+     * Returns list of all explicit parent languages for the given language.
      *
-     * @param string $lang
-     * @return array all parents, the lang itself is last
+     * English (en) is considered as the top implicit parent of all language packs
+     * and is not included in the returned list. The language itself is appended to the
+     * end of the list. The method is aware of circular dependency risk.
+     *
+     * @see self::populate_parent_languages()
+     * @param string $lang the code of the language
+     * @return array all explicit parent languages with the lang itself appended
      */
     public function get_language_dependencies($lang) {
-        if ($lang === 'en') {
-            return array();
-        }
-        if (!file_exists("$this->otherroot/$lang/langconfig.php")) {
-            return array();
-        }
-        $string = array();
-        include("$this->otherroot/$lang/langconfig.php");
-
-        if (empty($string['parentlanguage'])) {
-            return array($lang);
-        } else {
-            $parentlang = $string['parentlanguage'];
-            unset($string);
-            return array_merge($this->get_language_dependencies($parentlang), array($lang));
-        }
+        return $this->populate_parent_languages($lang);
     }
 
     /**
@@ -7222,6 +7212,46 @@ class core_string_manager implements string_manager {
             return -1;
         }
     }
+
+    /// End of external API ////////////////////////////////////////////////////
+
+    /**
+     * Helper method that recursively loads all parents of the given language.
+     *
+     * @see self::get_language_dependencies()
+     * @param string $lang language code
+     * @param array $stack list of parent languages already populated in previous recursive calls
+     * @return array list of all parents of the given language with the $lang itself added as the last element
+     */
+    protected function populate_parent_languages($lang, array $stack = array()) {
+
+        // English does not have a parent language.
+        if ($lang === 'en') {
+            return $stack;
+        }
+
+        // Prevent circular dependency (and thence the infinitive recursion loop).
+        if (in_array($lang, $stack)) {
+            return $stack;
+        }
+
+        // Load language configuration and look for the explicit parent language.
+        if (!file_exists("$this->otherroot/$lang/langconfig.php")) {
+            return $stack;
+        }
+        $string = array();
+        include("$this->otherroot/$lang/langconfig.php");
+
+        if (empty($string['parentlanguage']) or $string['parentlanguage'] === 'en') {
+            unset($string);
+            return array_merge(array($lang), $stack);
+
+        } else {
+            $parentlang = $string['parentlanguage'];
+            unset($string);
+            return $this->populate_parent_languages($parentlang, array_merge(array($lang), $stack));
+        }
+    }
 }
 
 
diff --git a/lib/tests/fixtures/langtest/aa/langconfig.php b/lib/tests/fixtures/langtest/aa/langconfig.php
new file mode 100644
index 00000000000..15ea4c8ab1d
--- /dev/null
+++ b/lib/tests/fixtures/langtest/aa/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'AA native name';
+$string['thislanguageint'] = 'AA international name';
diff --git a/lib/tests/fixtures/langtest/bb/langconfig.php b/lib/tests/fixtures/langtest/bb/langconfig.php
new file mode 100644
index 00000000000..3e7802c66f9
--- /dev/null
+++ b/lib/tests/fixtures/langtest/bb/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'Orphaned language with non-existing parent';
+$string['parentlanguage'] = 'bbparent';
diff --git a/lib/tests/fixtures/langtest/bc/langconfig.php b/lib/tests/fixtures/langtest/bc/langconfig.php
new file mode 100644
index 00000000000..cec2bf9693e
--- /dev/null
+++ b/lib/tests/fixtures/langtest/bc/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'My parent is orphaned language';
+$string['parentlanguage'] = 'bb';
diff --git a/lib/tests/fixtures/langtest/cda/langconfig.php b/lib/tests/fixtures/langtest/cda/langconfig.php
new file mode 100644
index 00000000000..e9af9cadfe4
--- /dev/null
+++ b/lib/tests/fixtures/langtest/cda/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'Circular dependency A';
+$string['parentlanguage'] = 'cdc';
diff --git a/lib/tests/fixtures/langtest/cdb/langconfig.php b/lib/tests/fixtures/langtest/cdb/langconfig.php
new file mode 100644
index 00000000000..e3f4c629320
--- /dev/null
+++ b/lib/tests/fixtures/langtest/cdb/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'Circular dependency B';
+$string['parentlanguage'] = 'cda';
diff --git a/lib/tests/fixtures/langtest/cdc/langconfig.php b/lib/tests/fixtures/langtest/cdc/langconfig.php
new file mode 100644
index 00000000000..42be227c51c
--- /dev/null
+++ b/lib/tests/fixtures/langtest/cdc/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'Circular dependency C';
+$string['parentlanguage'] = 'cdb';
diff --git a/lib/tests/fixtures/langtest/de/langconfig.php b/lib/tests/fixtures/langtest/de/langconfig.php
new file mode 100644
index 00000000000..843d7c1b4b3
--- /dev/null
+++ b/lib/tests/fixtures/langtest/de/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'Deutsch';
+$string['parentlanguage'] = 'en';
diff --git a/lib/tests/fixtures/langtest/de_du/langconfig.php b/lib/tests/fixtures/langtest/de_du/langconfig.php
new file mode 100644
index 00000000000..173d669c839
--- /dev/null
+++ b/lib/tests/fixtures/langtest/de_du/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'Deutsch - Du';
+$string['parentlanguage'] = 'de';
diff --git a/lib/tests/fixtures/langtest/de_kids/langconfig.php b/lib/tests/fixtures/langtest/de_kids/langconfig.php
new file mode 100644
index 00000000000..c5305ef4673
--- /dev/null
+++ b/lib/tests/fixtures/langtest/de_kids/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'Deutsch - Kids';
+$string['parentlanguage'] = 'de_du';
diff --git a/lib/tests/fixtures/langtest/sd/langconfig.php b/lib/tests/fixtures/langtest/sd/langconfig.php
new file mode 100644
index 00000000000..6551f7fc8b5
--- /dev/null
+++ b/lib/tests/fixtures/langtest/sd/langconfig.php
@@ -0,0 +1,4 @@
+<?php
+
+$string['thislanguage'] = 'Self dependency';
+$string['parentlanguage'] = 'sd';
diff --git a/lib/tests/string_test.php b/lib/tests/string_test.php
new file mode 100644
index 00000000000..7ca2d9b7630
--- /dev/null
+++ b/lib/tests/string_test.php
@@ -0,0 +1,107 @@
+<?php
+
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Unit tests for localization support in lib/moodlelib.php
+ *
+ * @package     core
+ * @category    test
+ * @copyright   2013 David Mudrak <david@moodle.com>
+ * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+global $CFG;
+require_once($CFG->libdir.'/moodlelib.php');
+
+/**
+ * Tests for the API of the string_manager
+ *
+ * @copyright 2013 David Mudrak <david@moodle.com>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class string_manager_test extends advanced_testcase {
+
+    public function test_string_manager_instance() {
+        global $CFG;
+        $this->resetAfterTest();
+
+        $otherroot = dirname(__FILE__).'/fixtures/langtest';
+        $stringman = testable_core_string_manager::instance($otherroot);
+        $this->assertTrue(in_array('string_manager', class_implements($stringman)));
+    }
+
+    public function test_get_language_dependencies() {
+        global $CFG;
+        $this->resetAfterTest();
+
+        $otherroot = dirname(__FILE__).'/fixtures/langtest';
+        $stringman = testable_core_string_manager::instance($otherroot);
+
+        // There is no parent language for 'en'.
+        $this->assertSame(array(), $stringman->get_language_dependencies('en'));
+        // Language with no parent language declared.
+        $this->assertSame(array('aa'), $stringman->get_language_dependencies('aa'));
+        // Language with parent language explicitly set to English (en < de).
+        $this->assertSame(array('de'), $stringman->get_language_dependencies('de'));
+        // Language dependency hierarchy (de < de_du < de_kids).
+        $this->assertSame(array('de', 'de_du', 'de_kids'), $stringman->get_language_dependencies('de_kids'));
+        // Language with the parent language misconfigured to itself (sd < sd).
+        $this->assertSame(array('sd'), $stringman->get_language_dependencies('sd'));
+        // Language with circular dependency (cda < cdb < cdc < cda).
+        $this->assertSame(array('cda', 'cdb', 'cdc'), $stringman->get_language_dependencies('cdc'));
+        // Orphaned language (N/A < bb).
+        $this->assertSame(array('bb'), $stringman->get_language_dependencies('bb'));
+        // Descendant of an orphaned language (N/A < bb < bc).
+        $this->assertSame(array('bb', 'bc'), $stringman->get_language_dependencies('bc'));
+    }
+}
+
+
+/**
+ * Helper class providing testable string_manager
+ *
+ * @copyright 2013 David Mudrak <david@moodle.com>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class testable_core_string_manager extends core_string_manager {
+
+    /**
+     * Factory method
+     *
+     * @param string $otherroot full path to the location of installed upstream language packs
+     * @param string $localroot full path to the location of locally customized language packs, defaults to $otherroot
+     * @param bool $usecache use application permanent cache
+     * @param array $translist explicit list of visible translations
+     * @param string $menucache the location of a file that caches the list of available translations
+     * @return testable_core_string_manager
+     */
+    public static function instance($otherroot, $localroot = null, $usecache = false, array $translist = array(), $menucache = null) {
+        global $CFG;
+
+        if (is_null($localroot)) {
+            $localroot = $otherroot;
+        }
+
+        if (is_null($menucache)) {
+            $menucache = $CFG->cachedir.'/languages';
+        }
+
+        return new testable_core_string_manager($otherroot, $localroot, $usecache, $translist, $menucache);
+    }
+}