Does anyone know how to change the default language in virt-manager?
Yes. 
What do you want to do?
What is your system’s language?
Why would you want to start a program with a different language than all the others?
I switched my language on my computer and virt-manager didn’t adapt.
I don’t know how you switched your systems language nor any system details like the Desktop you are using
the generic way is thusly:
locales are defined in /etc/locale.gen
uncomment every one you want (edit as root)
then run
sudo locale-gen
localectl list-locales
will list available locales
localectl set-locale de_DE.UTF-8
will set the german locale
provided it was made available
(use sudo)
if your system is english but you want to start a program (like virt-manager) in german
start it like:
LANG=de_DE.UTF-8 virt-manager
I do that often the other way around to see what the menu items are called in english
LANG=en_US.UTF-8 virt-manager
will start the prgram in english for me
Do I have to restart? It’s not working at the moment.
DE: KDE
Languages: Switched from French to English