How to install additional language packs on Ubuntu Gnome 15.04?

The terminal way

The terminal way is excellent described in answer of @GunnarHjalmarsson and in the answer of @Serg.

The GUI way

Start the Language Support via GNOME Shell or language-selector-gnome (GNOME 3.14) or gnome-language-selector (GNOME 3.16) in a terminal.

Click Install / Remove Languages… and select one or more languages to install

Here's a few command line tools you might not have been aware of.

1. The good-old apt-get!

   Search with apt-cache search language-pack-gnome-xx, there xx is the code for language. In your case, if you want russian – apt-cache search language-pack-gnome-ru. There's two packages, actually, language-pack-gnome-ru and language-pack-gnome-ru-base. Running sudo apt-get install language-pack-gnome-ru tells me:

   The following extra packages will be installed:
     firefox-locale-ru language-pack-gnome-ru-base language-pack-ru
     language-pack-ru-base
   The following NEW packages will be installed:
     firefox-locale-ru language-pack-gnome-ru language-pack-gnome-ru-base
     language-pack-ru language-pack-ru-base
   

   So as you can see, base package gets installed automatically – no need to worry about that.

2. Long time ago I had to write a script that automatically sets language input and turns out gsettings does that quite well.

   Run gsettings get org.gnome.desktop.input-sources sources to get your selection for current input sources available. If you want to add a language source, here's my example of adding Spanish to my list:

   gsettings set org.gnome.desktop.input-sources  sources "[('xkb', 'us'),('xkb','es'),('xkb','zh')]" 
   

   Notice the comas and single quotes, as well as double quotes. Very important for syntax.

3. check-language-support command.

   This neat little command will help you find out which language packs you are missing, you can target a specific language with -l or --language=LANG flags, as well as --show-installed. Neat, isn't it?

I'm aware of two ways. Either you install Language Support:

sudo apt-get install language-selector-gnome

and do it from there. Optionally you can just run these commands in a terminal window:

sudo mkdir /usr/share/locale-langpack/ru
sudo apt-get install $(check-language-support)