It appears that the root cause of this issue is that /usr/share/language-selector/data/languagelist is missing or incomplete. This file can be found in the package language-selector-common. A quick peek at
dpkg -l *language-selector* shows that the following should have been installed by default.
ii language-selector-common 0.165.4 all Language selector for Ubuntu
ii language-selector-gnome 0.165.4 all Language selector for Ubuntu
sudo apt install --reinstall language-selector-common will likely do the trick. If not you may need to purge and then reinstall with
sudo apt-get purge language-selector-common be aware that this approach will remove both language-selector-common and language-selector-gnome so we have to reinstall both of them with
sudo apt-get install language-selector-common language-selector-gnome
Open Language support from the dash:
Click install/remove languages.
Scroll to the language you wish to install, tick the box and click apply changes.
Support for your chosen language will be installed:
Now that you've installed the language in your system, each user (this means you) can select it as an option in text entry settings as shown below:
Hit the + button to bring up the menu as shown below:
Now you can select French as a text entry language.
You may wish to also check the keyboard layout chart.
Tested on Ubuntu 16.04