language-pack-* packages contain translations for the user interfaces of the various applications found in Ubuntu, mostly in the form of gettext message catalogues.
If you configure your user account to use a locale covered by the language pack, the user interface should then be displayed in that language when you next log in (few applications support changing their UI language while they are running).
In most cases, you wouldn't install the
language-pack-* packages directly, but instead install the corresponding
language-support-* package, which will give you other resources used by the language (e.g. spell checking dictionaries).
If you want to change the language settings of your Ubuntu system, the easiest method is with the
Language Support control panel found under