TS file language property is not changed on write
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"
Thanks for letting me know.
a fix (or workaround) has been merged, awaiting the next release.