There seems to be a typo in gtkfilechooserdefault.c. See the following patch:
diff --git a/gtk/gtkfilechooserdefault.c b/gtk/gtkfilechooserdefault.c
index 7f52872..2438b8b 100644
--- a/gtk/gtkfilechooserdefault.c
+++ b/gtk/gtkfilechooserdefault.c
@@ -769,7 +769,7 @@ static void
 error_filename_to_long_dialog (GtkFileChooserDefault *impl)
 {
   error_message (impl,
-                 _("Cannot create file as the filename is to long"),
+                 _("Cannot create file as the filename is too long"),
                  _("Try using a shorter name."));
 }