diff --git a/core/i18n.php b/core/i18n.php
index 7097d2fb4..42ed4d127 100755
--- a/core/i18n.php
+++ b/core/i18n.php
@@ -1603,7 +1603,9 @@ class i18n extends Object {
 	 * @return string Long locale, e.g. "en_US"
 	 */
 	static function get_locale_from_lang($lang) {
-		if(isset(self::$likely_subtags[$lang])) {
+		if(preg_match('/\-|_/', $lang)) {
+			return $lang;
+		} else if(isset(self::$likely_subtags[$lang])) {
 			return self::$likely_subtags[$lang];
 		} else {
 			return $lang . '_' . strtoupper($lang);
diff --git a/tests/i18n/i18nTest.php b/tests/i18n/i18nTest.php
index 51b53edee..bdbd8fca5 100644
--- a/tests/i18n/i18nTest.php
+++ b/tests/i18n/i18nTest.php
@@ -198,6 +198,12 @@ class i18nTest extends SapphireTest {
 		
 		i18n::set_locale($oldLocale);
 	}
+	
+	function testGetLocaleFromLang() {
+		$this->assertEquals('en_US', i18n::get_locale_from_lang('en'));
+		$this->assertEquals('de_DE', i18n::get_locale_from_lang('de_DE'));
+		$this->assertEquals('xy_XY', i18n::get_locale_from_lang('xy'));
+	}
 }
 
 class i18nTest_DataObject extends DataObject implements TestOnly {