It may be that your MST is applying successfully, but updating the ProductLanguage of the MSI is simply having no effect. There does not appear to be anything within the MSI setup to take advantage of this property.
Since you are already comfortable making an MST, your best option is to use Master Packager (the free version will do) to create an action to apply HKCU to all users:
https://www.masterpackager.com/master-packager
Since you are already comfortable making an MST, your best option is to use Master Packager (the free version will do) to create an action to apply HKCU to all users:
https://www.masterpackager.com/master-packager
![](https://www.masterpackager.com/uploads/file_archive/b_20_tips_predfinedca.gif)