Hallo,
how can I change the system language in QubesOS?
New TemlateVMs and AppVMs shall be created in german language and the whole system appearance shall be changed to german language?
The Documentation and Introduction is very well done, just this information I could not find.
Thank you for your help in advance!
Kris
In dom0, install the German language. In a dom0 terminal, execute sudo qubes-dom0-update langpacks-de.
Then reboot.
Before logging in, the task panel will show - usually far to the right - the current language, e.g. C.UTF-8. Clicking on this value will open a menu where you can select the GUI language.
This need only be done once; the selected language survives logging out and reboot.
Thank you very much!
2 more questions, just to understand the logic:
Does this procedure include language-settings of in the future created TemplateVMs only, I guess?
How can I change language-settings in already created TemplatVMs?
That’s just the interface language of Qubes, i.e. dom0.
To change the language of existing templates, you have to install the language packs in these templates.
For Fedora-based templates, this is done via sudo dnf install langpacks-de, and for debian via sudo apt-get install language-pack-de language-pack-gnome-de language-pack-de-base language-pack-gnome-de-base.
After installing a language, it has to be selected/enabled via the settings of the template.
New templates will be installed in their default language, usually English, and they have to be changed just like existing templates.
This could be alleviated by coping a “clean” template with nothing but the language packs before starting to create a new template–clone the “clean Deutsch” template and start from there.
However, that won’t help when you need a totally new template, (e.g., Debian 12 when it comes out); you’ll have to create debian-12-de and regenerate all other templates from that. (Many people do that with scripts or manage in some way I don’t quite understand with salt.)
Thank you very much. I think I will stay with installing the language packs manually.
I think I now understand the system:
When I do general changes in TemplateVMs then I can also search for further informations in the VMs-distro-forums if not found here. And if I do not find an answer I know that here are helpful users 
Hallo, I did exactly what you told and my system language still didn’t change. Has anything changed since then? Need some additional steps?
Sorry! Actually it worked. You just need to reboot system again after you changed system language before login in.
But where are those settings? I installed language pack for Fedora Template, restarted and its language didn’t change. Then I looked its settings and found no settings related to language. There was parameter “Allow UTF-8 window titles” but changing it to “Allow” didn’t help. Also looked in “Qubes Global Settings” and in “Qubes Template Manager” - there are no settings related to language.
Also tried to install language packs for Debian Templates (my defaults) and Whonixes and each time it was saying that it can’t locate language pack. For test tried to install exactly that German pack you said and it can’t locate it too. Probably it means the language pack names were changed. How to find out what new names are?
Unfortunately, that depends on the Linux variant in the template:
For Fedora, you have to change the file /etc/locale.conf to, e.g. LANG=de_DE.UTF-8 or so, and restart the VM.
For Debian, the command is sudo dpkg-reconfigure locales.
I never used Fedora and when I faced it in Qubes, I decided to give it a try. To see how it differs from Debian. So, honestly I have no idea how to change that file. Don’t know the needed command. And what means “to change”? In Debian I enter the command then it’s like I choose the needed language from the list to install it (or to turn it on, don’t know for sure). Anyway, if there need only one command in terminal, then could you give it? Though I already see that for Fedora I probably need to learn another whole set of commands 
so not sure why the developers chose this particular operating system if it’s kinda not so popural among users.
You can edit the file in terminal:
sudo nano /etc/locale.conf