How can I export all untranslated entries?

This can be done with the Filter feature:

• Click the small funnel icon at the top-right of the translation id list.
• Choose Show only untranslated.
• Press Ctrl+A (or Cmd+A on macOS) to select all visible translation IDs.
• Click Export, then enable Export selected translation ids only.

For more complex filtering, use the "Set Filter" funnel icon in the toolbar which opens a dialog with more advanced options.