diff --git a/core/lib/Drupal/Component/Transliteration/PHPTransliteration.php b/core/lib/Drupal/Component/Transliteration/PHPTransliteration.php
index a495ee8448b1..80982d27f61a 100644
--- a/core/lib/Drupal/Component/Transliteration/PHPTransliteration.php
+++ b/core/lib/Drupal/Component/Transliteration/PHPTransliteration.php
@@ -172,7 +172,8 @@ class PHPTransliteration implements TransliterationInterface {
    * PHPTransliteration::$dataDirectory. These files should set up an array
    * variable $overrides with an element whose key is $langcode and whose value
    * is an array whose keys are character codes, and whose values are their
-   * transliterations in this language.
+   * transliterations in this language. The character codes can be for any valid
+   * Unicode character, independent of the number of bytes.
    *
    * @param $langcode
    *   Code for the language to read.
@@ -200,7 +201,8 @@ class PHPTransliteration implements TransliterationInterface {
    * hexidecimal notation) in PHPTransliteration::$dataDirectory. These files
    * should set up a variable $bank containing an array whose numerical indices
    * are the remaining two bytes of the character code, and whose values are the
-   * transliterations of these characters into US-ASCII.
+   * transliterations of these characters into US-ASCII. Note that the maximum
+   * Unicode character that can be encoded in this way is 4 bytes.
    *
    * @param $bank
    *   First two bytes of the Unicode character, or 0 for the ASCII range.
diff --git a/core/lib/Drupal/Core/Transliteration/PHPTransliteration.php b/core/lib/Drupal/Core/Transliteration/PHPTransliteration.php
index 951ce1e526c2..6af4708379f7 100644
--- a/core/lib/Drupal/Core/Transliteration/PHPTransliteration.php
+++ b/core/lib/Drupal/Core/Transliteration/PHPTransliteration.php
@@ -13,6 +13,7 @@ use Drupal\Component\Transliteration\PHPTransliteration as BaseTransliteration;
  * Enhances PHPTransliteration with an alter hook.
  *
  * @ingroup transliteration
+ * @see hook_transliteration_overrides_alter()
  */
 class PHPTransliteration extends BaseTransliteration {
 
diff --git a/core/modules/system/language.api.php b/core/modules/system/language.api.php
index ed81b9235be5..7c7375f0db72 100644
--- a/core/modules/system/language.api.php
+++ b/core/modules/system/language.api.php
@@ -177,11 +177,14 @@ function hook_language_fallback_candidates_alter(array &$fallback_candidates) {
  * vs. initial capital letter only) is not taken into account, and in
  * transliterations of capital letters that result in two or more letters, by
  * convention only the first is capitalized in the Drupal transliteration
- * result. So, the process has limitations; however, since the reason for
- * transliteration is typically to create machine names or file names, this
- * should not really be a problem. After transliteration, other transformation
- * or validation may be necessary, such as converting spaces to another
- * character, removing non-printable characters, lower-casing, etc.
+ * result. Also, only Unicode characters of 4 bytes or less can be
+ * transliterated in the base system; language-specific overrides can be made
+ * for longer Unicode characters. So, the process has limitations; however,
+ * since the reason for transliteration is typically to create machine names or
+ * file names, this should not really be a problem. After transliteration,
+ * other transformation or validation may be necessary, such as converting
+ * spaces to another character, removing non-printable characters,
+ * lower-casing, etc.
  *
  * Here is a code snippet to transliterate some text:
  * @code
@@ -196,13 +199,20 @@ function hook_language_fallback_candidates_alter(array &$fallback_candidates) {
  * Drupal Core provides the generic transliteration character tables and
  * overrides for a few common languages; modules can implement
  * hook_transliteration_overrides_alter() to provide further language-specific
- * overrides. Modules can also completely override the transliteration classes
- * in \Drupal\Core\CoreBundle.
+ * overrides (including providing transliteration for Unicode characters that
+ * are longer than 4 bytes). Modules can also completely override the
+ * transliteration classes in \Drupal\Core\CoreBundle.
  */
 
 /**
  * Provide language-specific overrides for transliteration.
  *
+ * If the overrides you want to provide are standard for your language, consider
+ * providing a patch for the Drupal Core transliteration system instead of using
+ * this hook. This hook can be used temporarily until Drupal Core's
+ * transliteration tables are fixed, or for sites that want to use a
+ * non-standard transliteration system.
+ *
  * @param array $overrides
  *   Associative array of language-specific overrides whose keys are integer
  *   Unicode character codes, and whose values are the transliterations of those
diff --git a/core/modules/system/lib/Drupal/system/Tests/Transliteration/TransliterationTest.php b/core/modules/system/lib/Drupal/system/Tests/Transliteration/TransliterationTest.php
index d717d8b9a1ea..7630b0bf1fb1 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Transliteration/TransliterationTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Transliteration/TransliterationTest.php
@@ -43,6 +43,13 @@ class TransliterationTest extends DrupalUnitTestBase {
     // This is a Canadian Aboriginal character like a triangle. See
     // http://www.unicode.org/charts/PDF/U1400.pdf
     $four_byte = html_entity_decode('ᐑ', ENT_NOQUOTES, 'UTF-8');
+    // These are two Gothic alphabet letters. See
+    // http://en.wikipedia.org/wiki/Gothic_alphabet
+    // They are not in our tables, but should at least give us '?' (unknown).
+    $five_byte = html_entity_decode('𐌰𐌸', ENT_NOQUOTES, 'UTF-8');
+    // Five-byte characters do not work in MySQL, so make a printable version.
+    $five_byte_printable = '𐌰𐌸';
+
     $cases = array(
       // Each test case is (language code, input, output).
       // Test ASCII in English.
@@ -55,6 +62,8 @@ class TransliterationTest extends DrupalUnitTestBase {
       // directly from the data files.
       array('fr', $three_byte, 'c'),
       array('fr', $four_byte, 'wii'),
+      // Test 5-byte characters.
+      array('en', $five_byte, '??', $five_byte_printable),
       // Test a language with no overrides.
       array('en', $two_byte, 'A O U A O aouaohello'),
       // Test language overrides provided by core.
@@ -64,9 +73,10 @@ class TransliterationTest extends DrupalUnitTestBase {
       array('dk', $random, $random),
       array('kg', $three_byte, 'ts'),
       // Test the language override hook in the test module, which changes
-      // the transliteration of Ä to Z.
+      // the transliteration of Ä to Z and provides for the 5-byte characters.
       array('zz', $two_byte, 'Z O U A O aouaohello'),
       array('zz', $random, $random),
+      array('zz', $five_byte, 'ATh', $five_byte_printable),
       // Test strings in some other languages.
       // Turkish, provided by drupal.org user Kartagis.
       array('tr', 'Abayı serdiler bize. Söyleyeceğim yüzlerine. Sanırım hepimiz aynı şeyi düşünüyoruz.', 'Abayi serdiler bize. Soyleyecegim yuzlerine. Sanirim hepimiz ayni seyi dusunuyoruz.'),
@@ -78,10 +88,11 @@ class TransliterationTest extends DrupalUnitTestBase {
 
     foreach($cases as $case) {
       list($langcode, $original, $expected) = $case;
+      $printable = (isset($case[3])) ? $case[3] : $original;
       $transliterator_class = new PHPTransliteration();
       $actual = $transliterator_class->transliterate($original, $langcode);
       $this->assertIdentical($actual, $expected, format_string('@original transliteration to @actual is identical to @expected for language @langcode in new class instance.', array(
-        '@original' => $original,
+        '@original' => $printable,
         '@langcode' => $langcode,
         '@expected' => $expected,
         '@actual' => $actual,
@@ -89,7 +100,7 @@ class TransliterationTest extends DrupalUnitTestBase {
 
       $actual = $transliterator_service->transliterate($original, $langcode);
       $this->assertIdentical($actual, $expected, format_string('@original transliteration to @actual is identical to @expected for language @langcode in service instance.', array(
-        '@original' => $original,
+        '@original' => $printable,
         '@langcode' => $langcode,
         '@expected' => $expected,
         '@actual' => $actual,
diff --git a/core/modules/system/tests/modules/transliterate_test/transliterate_test.module b/core/modules/system/tests/modules/transliterate_test/transliterate_test.module
index ee71069db368..636fde8a188d 100644
--- a/core/modules/system/tests/modules/transliterate_test/transliterate_test.module
+++ b/core/modules/system/tests/modules/transliterate_test/transliterate_test.module
@@ -12,5 +12,9 @@ function transliterate_test_transliteration_overrides_alter(&$overrides, $langco
   if ($langcode == 'zz') {
     // The default transliteration of Ä is A, but change it to Z for testing.
     $overrides[0xC4] = 'Z';
+    // Also provide transliterations of two 5-byte characters from
+    // http://en.wikipedia.org/wiki/Gothic_alphabet.
+    $overrides[0x10330] = 'A';
+    $overrides[0x10338] = 'Th';
   }
 }