At the stage screen, users can now import strings from an uploaded file. The only supported format at the moment is standard PHP format used by Moodle itself - that is a definition of $string array. More formats (like XLIFF or other XML based one) are expected to be supported in the future.
This means that AMOS actually supports offline translations now. Just download the English language pack from the langpack download page, translate it offline and import the files back. You can import one file at the time. We can add support for bulk importing from a ZIP file if there is strong demand later.