How to add/set language in GNOME?

Welcome @His_Turdness,

to change it check https://wiki.archlinux.org/index.php/Locale

You need to modify the file /etc/locale.gen
Uncomment (erase) the # to the languages You want

then run as root

locale-gen    

restart x and now you can chose more languages in region and language in Gnome.