I am looking to delete fonts through font manager and like everyone I have many fonts for languages that I do not write or speak. Can system fonts be safely deleted or will it destabilise things?
Yes, they can.
Please, have a look at
Unused fonts can be fully removed via the Synaptic package manager.
I do not understand why Ubuntu (and not only ubuntu-mate) install so much fonts that does not have any link with the language choices made in the install process...