When I installed Ubuntu GNOME 15.10 with GNOME 3.18 I selected "English (UK)" as my language. However when I then installed other programs such as GIMP and LibreOffice, and used Firefox I found that language for spell checkers etc was "English (US)", and this would be because the default is that and the correct language packs for all these programs have not been installed.

I have found that the only way to install all the needed language packs is to install the language-selector-gnome. Because when started it will say "Some translations or writing aids available for your chosen languages are not installed yet. Do you want to install them now?" and give you the option to install them, however when starting the gnome-control-center and going to the "Region & Language" section there is no such prompt, and in fact no way to install additional language packs for the chosen language.

And as the language-selector-gnome does actually break some stuff and is a Unity thing, one should need to install it on GNOME, there should be an easy way of doing it through the built-in tools made available to the user.

I initially reported this issue here but thought I should also do so upstream: https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/1525281