The DocumentPart containing "<TS version=\"2.1\" language=\"en_US\">\n" is not updated with the target locale. e.g. if writing a translated file_de.ts, it should be "<TS version=\"2.1\" language=\"de\">\n"
Are you working on it? Besides the other bugs, this relates also to the problem described in ZNTA-2581 which is a key feature of Zanata.
a fix (or workaround) has been merged, awaiting the next release.
Thanks for letting me know.