Manually delete unused languages from macOS
The language files for each application are stored in

APPL.app/Contents/Resources/LANG.lproj


Removing them for non-standard applications is easy (especially from Terminal), for standard applications you need to disable SIP first.

The space gained by removing them may be small though, they use about 1.3 GB in my Applications folder with about 350 applications installed. Also you need to redo this with every upgrade. You probably gain more by removing a few unused non-standard applications.