Welsh is cy_GB and not uk_UA.

functions/i18n.php

@@ -1354,9 +1354,9 @@ $languages['uk_UA']['NAME']    = 'Ukrainian';
 $languages['uk_UA']['CHARSET'] = 'koi8-u';
 $languages['uk']['ALIAS'] = 'uk_UA';
 
-$languages['uk_UA']['NAME']    = 'Welsh';
-$languages['uk_UA']['CHARSET'] = 'iso-8859-1';
-$languages['uk']['ALIAS'] = 'cy_GB';
+$languages['cy_GB']['NAME']    = 'Welsh';
+$languages['cy_GB']['CHARSET'] = 'iso-8859-1';
+$languages['cy']['ALIAS'] = 'cy_GB';
 
 
 // Right to left languages