TS files normally have the language and sourcelanguage attributes set for the target and source languages.
For example,

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1" language="fr_FR" sourcelanguage="en_US" >
</TS>

The qt_add_translations() command is generating .ts files which are missing the language and sourcelanguage attributes.
For example,

<TS version="2.1">
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1">
</TS>

Same behavior also observed when generating a plurals-only .ts file via the qt_add_lupdate(PLURAL_TS_FILE file.ts) and qt_add_translations(PLURAL_TS_FILE file.ts) commands.

To reproduce issue, build attached project with update_translations.