This normally should be possible by adding the preferred language locale to the environment variable LC_ALL
Example:
$ LC_ALL=zh_CN.utf8 gnome-clocks
should open gnome-clocks in chinese locale.
How this is done in GNOME Builder is not clear. There is a way to add environment variables to the build environment, but I don’t see an option for adding environment variables to runtime environment, which is what is needed here.