I18n: translate untitled file names

When saving, the default file name is "untitled" regardless of
selected language. This can be translated, like many graphical
applications do.

This applies to:
- blend file
- alembic file
- collada file
- obj file
- usd file
- rendered image
- grease pencil export
- subtitles export
- other Python exports through ExportHelper

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D15868

source/blender/editors/space_sequencer/sequencer_edit.c

@@ -3093,7 +3093,7 @@ static int sequencer_export_subtitles_invoke(bContext *C,
     char filepath[FILE_MAX];
 
     if (BKE_main_blendfile_path(bmain)[0] == '\0') {
-      BLI_strncpy(filepath, "untitled", sizeof(filepath));
+      BLI_strncpy(filepath, DATA_("untitled"), sizeof(filepath));
     }
     else {
       BLI_strncpy(filepath, BKE_main_blendfile_path(bmain), sizeof(filepath));