Thank you for your answer.
That's exactly what we have done until now. But the new auto update package use a MSI installer:
PDQ-Starting with 65.0, this package now uses an MSI-based installer and MST transform. For more information: https://support.mozilla.org/en-US/kb/firefox-customization-msi-installers
As mentioned on the support site of Mozilla:
To install Firefox MSI in a different language, please replace the string after lang= with the language code you need.
But I cannot find the language setting in the transform file. Can I simply add this parameter to the msi command? If yes, it is not possible to edit an Auto Deployment package in PDQ Deploy. Do we lose the possibility of Auto Upgrade Deployments for Firefox?