Fix i18n language code validation to include `-`

cmd/handlers.go

@@ -33,7 +33,7 @@ type pagination struct {
 
 var (
 	reUUID     = regexp.MustCompile("^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$")
-	reLangCode = regexp.MustCompile("[^a-zA-Z_0-9]")
+	reLangCode = regexp.MustCompile("[^a-zA-Z_0-9\\-]")
 )
 
 // registerHandlers registers HTTP handlers.