Much the same as previous posts
Are you also a newbie to linux as well as Gimp? You might be better off asking on an ubuntu forum.
This is my Kubuntu 16.04 laptop and Gimp 2.8.18
You will probably be using the Gimp 2.8.16 from the ubuntu xenial repo, but that is ok.
When you say you can 'not get anything but a sans font' does this mean the drop down list in the font tool options is empty?
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If it is, look in Edit -> Preferences -> Folders -> Fonts and there should be two locations, one in your home directory and the other in the gimp root share. Both of these are initially empty. All you get are the system fonts. If you added folders containing fonts you need to add the location there as well.
First thing to try, put some of those fonts into your home gimp profile ~/.gimp-2.8/fonts This is a hidden folder. Turn on
show hidden files in your file manager (shortcut alt. (alt dot))
Start Gimp. Do you see those fonts in the font list.
No? then something is really wrong with your installation.
You could try deleting your Gimp profile ~/.gimp-2.8 then reinstalling Gimp. Sometimes fixes a broken package.