10 years ago
As remoulder points out you can already install a font(ttf/otf) file by double clicking on it (gnome 2.32 in my case) and therefore I mark this as implemented. It's not under the context menu as you wished for but it's working.
It would help a lot with a link to the duplicate.
In Mint all you have to do is double click the font and it will open in a viewer that has an install button and it will easily install your fonts.
From Gnome 2.30 release notes "New fonts can be installed by double-clicking the font in Nautilus" assuming you are using Isadora.
What you are suggesting needs to be done upstream, the mint team doesn't have the resources to do things like this.