How can I install one language by command-line
Solution 1:
For desktop you can
sudo apt-get install language-pack-[cod] language-pack-gnome-[cod] language-pack-[cod]-base language-pack-gnome-[cod]-base
But each applications can have its language pack. To know the packages to be installed try:
check-language-support -l [cod]
For example:
check-language-support -l fr
Solution 2:
Auto install all dependecies:
sudo apt-get -y install `check-language-support -l fr`
where fr is the desired language code.