To make the new font the desktop default either the theme could be adapted to load the font, or "sans" which is an alias for a list of preferred fonts could be updated to include ubuntu in the preferences list, I think that is in the fontconfig package and would be a change to /etc/fonts/conf.avail/45-latin.conf and possibly /etc/fonts/conf.avail/40-nonlatin.conf (although I am not familiar with how the whole fontconfig thing works). Anyhow at the moment fontconfig maps sans to DejaVuSans by default.
using sfddiff ttf-dejavu/DejaVuSans.ttf ubuntu-private-fonts/UbuntuRegular.ttf to compare the glyphs gives the attached output (it segfaulted, but not before it did the glyph comparison.)
To make the new font the desktop default either the theme could be adapted to load the font, or "sans" which is an alias for a list of preferred fonts could be updated to include ubuntu in the preferences list, I think that is in the fontconfig package and would be a change to /etc/fonts/ conf.avail/ 45-latin. conf and possibly /etc/fonts/ conf.avail/ 40-nonlatin. conf (although I am not familiar with how the whole fontconfig thing works). Anyhow at the moment fontconfig maps sans to DejaVuSans by default. DejaVuSans. ttf ubuntu- private- fonts/UbuntuReg ular.ttf to compare the glyphs gives the attached output (it segfaulted, but not before it did the glyph comparison.)
using sfddiff ttf-dejavu/