translate_document() should seek(0, 0) the output file after being done

[Nama]

After the output_document is finished, the pointer should be set to 0, 0 to make further processing possible.

result = BytesIO()
translator.translate_document(input_document=source_file, output_document=result, target_lang=target_lang, filename=source_file.filename)
result.seek(0, 0)  # required, else flask wont send the file
return flask.send_file(result, download_name=source_file.filename, as_attachment=True)