diff --git a/src/intl/libintl.c b/src/intl/libintl.c
index f49306694..3d66e9672 100644
--- a/src/intl/libintl.c
+++ b/src/intl/libintl.c
@@ -71,7 +71,7 @@ struct language languages[] = {
  * other side, it's ISO639 for gettext as well etc. So what?  --pasky */
 
 int
-iso639_to_language(char *iso639)
+iso639_to_language(const char *iso639)
 {
 	char *l = stracpy(iso639);
 	char *p;
diff --git a/src/intl/libintl.h b/src/intl/libintl.h
index e7667a46e..3f4640d6a 100644
--- a/src/intl/libintl.h
+++ b/src/intl/libintl.h
@@ -138,14 +138,14 @@ extern char *EL_LANGUAGE;
 
 struct language {
 	char *name;
-	char *iso639;
+	const char *iso639;
 };
 
 extern struct language languages[];
 
 /* These two calls return 1 (english) if the code/name wasn't found. */
 extern int name_to_language(const char *name);
-extern int iso639_to_language(char *iso639);
+extern int iso639_to_language(const char *iso639);
 
 extern char *language_to_name(int language);
 extern char *language_to_iso639(int language);