Using qt_add_translations does not update .ts file, even when existing translations are changed

without the target language specified in the .ts files, the changes in the existing translations are not detected by the .ts file....

it only works when a language tag is added: 

<TS version="2.1" language="ja_JP"> then we see :

    <message>
<location filename="../../../../ImSim/Constants.qml" line="23"/>
<source>Stray Light</source>
<translation type="unfinished"></translation>
</message>
<message>
<source>Stray Light Settings</source>
<translation type="obsolete">迷光セッティング</translation>
</message>