I have a rather clean installation of Ubuntu 19.04 on which I have downloaded a Google font (TTF-format), opened it in Font Viewer, and clicked install. In Font Manager I now find the font listed under "User" (not under "System"). The font is also in my ~/.local/share/fonts folder. I am able to use the font in apps like GIMP.
However, apps like LibreOffice and Inkscape still don't show the font, not even after reboot. Does anyone know a solution for this? Thanks for helping me out!