diff --git a/phpBB/includes/functions_user.php b/phpBB/includes/functions_user.php
index dd96530265..e8db807154 100644
--- a/phpBB/includes/functions_user.php
+++ b/phpBB/includes/functions_user.php
@@ -1421,6 +1421,31 @@ function validate_match($string, $optional = false, $match = '')
 	return false;
 }
 
+/**
+* Validate Language Pack ISO Name
+*
+* Tests whether a language name is valid and installed
+*
+* @param string $lang_iso	The language string to test
+*
+* @return bool|string		Either false if validation succeeded or
+*							a string which will be used as the error message
+*							(with the variable name appended)
+*/
+function validate_language_iso_name($lang_iso)
+{
+	global $db;
+
+	$sql = 'SELECT lang_id
+		FROM ' . LANG_TABLE . "
+		WHERE lang_iso = '" . $db->sql_escape($lang_iso) . "'";
+	$result = $db->sql_query($sql);
+	$lang_id = (int) $db->sql_fetchfield('lang_id');
+	$db->sql_freeresult($result);
+
+	return ($lang_id) ? false : 'WRONG_DATA';
+}
+
 /**
 * Check to see if the username has been taken, or if it is disallowed.
 * Also checks if it includes the " character, which we don't allow in usernames.
diff --git a/phpBB/includes/ucp/ucp_prefs.php b/phpBB/includes/ucp/ucp_prefs.php
index b1f96356d5..51262c2289 100644
--- a/phpBB/includes/ucp/ucp_prefs.php
+++ b/phpBB/includes/ucp/ucp_prefs.php
@@ -65,7 +65,7 @@ class ucp_prefs
 
 					$error = validate_data($data, array(
 						'dateformat'	=> array('string', false, 1, 30),
-						'lang'			=> array('match', false, '#^[a-z0-9_\-]{2,}$#i'),
+						'lang'			=> array('language_iso_name'),
 						'tz'			=> array('num', false, -14, 14),
 					));
 
diff --git a/phpBB/includes/ucp/ucp_register.php b/phpBB/includes/ucp/ucp_register.php
index eaad4a7093..71374a9381 100644
--- a/phpBB/includes/ucp/ucp_register.php
+++ b/phpBB/includes/ucp/ucp_register.php
@@ -57,7 +57,7 @@ class ucp_register
 		{
 			$use_lang = ($change_lang) ? basename($change_lang) : basename($user_lang);
 
-			if (file_exists($user->lang_path . $use_lang . '/'))
+			if (!validate_language_iso_name($use_lang))
 			{
 				if ($change_lang)
 				{
@@ -211,7 +211,7 @@ class ucp_register
 					array('email')),
 				'email_confirm'		=> array('string', false, 6, 60),
 				'tz'				=> array('num', false, -14, 14),
-				'lang'				=> array('match', false, '#^[a-z_\-]{2,}$#i'),
+				'lang'				=> array('language_iso_name'),
 			));
 
 			if (!check_form_key('ucp_register'))