Changing the base OS language

You can't change the base OS language on the fly during an in-place upgrade. If your current base OS is for example en-US with language packs installed for de-DE, you are not able to upgrade your system with a new base OS with de-DE as the base language. If you try to do so, the in-place upgrade process will not migrate your applications. There are some hacks on the internet to install all the language content of the new base language you want to change to, booting into Windows PE and setting the new international settings, and getting an OS that looks like it is specific to this changed language and will be accepted by the in-place update routine. But be warned: this is not officially supported in terms of not being tested by Microsoft, and therefore, you could run into trouble and not be able to get official help.