diff --git a/filter/mathjaxloader/filter.php b/filter/mathjaxloader/filter.php
index 5aa51263f9a..a6010bfbcad 100644
--- a/filter/mathjaxloader/filter.php
+++ b/filter/mathjaxloader/filter.php
@@ -36,55 +36,39 @@ class filter_mathjaxloader extends moodle_text_filter {
      * @return string The MathJax language code.
      */
     public function map_language_code($moodlelangcode) {
-        $mathjaxlangcodes = array('br',
-                                  'cdo',
-                                  'cs',
-                                  'da',
-                                  'de',
-                                  'en',
-                                  'eo',
-                                  'es',
-                                  'fa',
-                                  'fi',
-                                  'fr',
-                                  'gl',
-                                  'he',
-                                  'ia',
-                                  'it',
-                                  'ja',
-                                  'ko',
-                                  'lb',
-                                  'mk',
-                                  'nl',
-                                  'oc',
-                                  'pl',
-                                  'pt',
-                                  'pt-br',
-                                  'ru',
-                                  'sl',
-                                  'sv',
-                                  'tr',
-                                  'uk',
-                                  'zh-hans');
-        $exceptions = array('cz' => 'cs');
 
-        // First see if this is an exception.
-        if (isset($exceptions[$moodlelangcode])) {
-            $moodlelangcode = $exceptions[$moodlelangcode];
+        // List of language codes found in the MathJax/localization/ directory.
+        $mathjaxlangcodes = [
+            'ar', 'ast', 'bcc', 'bg', 'br', 'ca', 'cdo', 'ce', 'cs', 'cy', 'da', 'de', 'diq', 'en', 'eo', 'es', 'fa',
+            'fi', 'fr', 'gl', 'he', 'ia', 'it', 'ja', 'kn', 'ko', 'lb', 'lki', 'lt', 'mk', 'nl', 'oc', 'pl', 'pt',
+            'pt-br', 'qqq', 'ru', 'scn', 'sco', 'sk', 'sl', 'sv', 'th', 'tr', 'uk', 'vi', 'zh-hans', 'zh-hant'
+        ];
+
+        // List of explicit mappings and known exceptions (moodle => mathjax).
+        $explicit = [
+            'cz' => 'cs',
+            'pt_br' => 'pt-br',
+            'zh_tw' => 'zh-hant',
+            'zh_cn' => 'zh-hans',
+        ];
+
+        // If defined, explicit mapping takes the highest precedence.
+        if (isset($explicit[$moodlelangcode])) {
+            return $explicit[$moodlelangcode];
         }
 
-        // Now look for an exact lang string match.
+        // If there is exact match, it will be probably right.
         if (in_array($moodlelangcode, $mathjaxlangcodes)) {
             return $moodlelangcode;
         }
 
-        // Now try shortening the moodle lang string.
-        $moodlelangcode = preg_replace('/-.*/', '', $moodlelangcode);
-        // Look for a match on the shortened string.
-        if (in_array($moodlelangcode, $mathjaxlangcodes)) {
-            return $moodlelangcode;
+        // Finally try to find the best matching mathjax pack.
+        $parts = explode('_', $moodlelangcode, 2);
+        if (in_array($parts[0], $mathjaxlangcodes)) {
+            return $parts[0];
         }
-        // All failed - use english.
+
+        // No more guessing, use English.
         return 'en';
     }
 
diff --git a/filter/mathjaxloader/tests/filter_test.php b/filter/mathjaxloader/tests/filter_test.php
new file mode 100644
index 00000000000..06eeb7989c3
--- /dev/null
+++ b/filter/mathjaxloader/tests/filter_test.php
@@ -0,0 +1,72 @@
+<?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/>.
+
+/**
+ * Provides the {@link filter_mathjaxloader_filter_testcase} class.
+ *
+ * @package     filter_mathjaxloader
+ * @category    test
+ * @copyright   2017 David Mudrák <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->dirroot.'/filter/mathjaxloader/filter.php');
+
+/**
+ * Unit tests for the MathJax loader filter.
+ *
+ * @copyright 2017 David Mudrak <david@moodle.com>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class filter_mathjaxloader_filter_testcase extends advanced_testcase {
+
+    /**
+     * Test the functionality of {@link filter_mathjaxloader::map_language_code()}.
+     *
+     * @param string $moodlelangcode the user's current language
+     * @param string $mathjaxlangcode the mathjax language to be used for the moodle language
+     *
+     * @dataProvider test_map_language_code_expected_mappings
+     */
+    public function test_map_language_code($moodlelangcode, $mathjaxlangcode) {
+
+        $filter = new filter_mathjaxloader(context_system::instance(), []);
+        $this->assertEquals($mathjaxlangcode, $filter->map_language_code($moodlelangcode));
+    }
+
+    /**
+     * Data provider for {@link self::test_map_language_code}
+     *
+     * @return array of [moodlelangcode, mathjaxcode] tuples
+     */
+    public function test_map_language_code_expected_mappings() {
+
+        return [
+            ['cz', 'cs'], // Explicit mapping.
+            ['cs', 'cs'], // Implicit mapping (exact match).
+            ['ca_valencia', 'ca'], // Implicit mapping of a Moodle language variant.
+            ['pt_br', 'pt-br'], // Explicit mapping.
+            ['en_kids', 'en'], // Implicit mapping of English variant.
+            ['de_kids', 'de'], // Implicit mapping of non-English variant.
+            ['es_mx_kids', 'es'], // More than one underscore in the name.
+            ['zh_tw', 'zh-hant'], // Explicit mapping of the Taiwain Chinese in the traditional script.
+            ['zh_cn', 'zh-hans'], // Explicit mapping of the Simplified Chinese script.
+        ];
+    }
+}