Hi Rosa,
You'll always want to create duplicates of your course and export the XLIFF file from each duplicate you need to translate. We talk more about that in this help guide. When your translation company transferred their work into the new XLIFF file, there still may have been something connecting it to the original version.
We'd love to have one of our support engineers investigate this translation issue further. Please reach out to us here and include the duplicate course's XLIFF file and the translation company's XLIFF file you are trying to import. Our engineers are ready to help.
Best regards,
Chris S.