Installing a second language in magento or Adding a new language in Magento is very easy.I am describing it with an example.Installing the French language
For this I search extension in magento Commerce for France language and copied the extension (magento-community/Locale_Mage_community_fr_FR). Now I logged in to magento connect manager of my website and installed this extension. Then I Create 2 store view one is for English and another one is for French .Be sure to use en in the code field for English Store view and Fr for France store view. Now go to System->configuration, from Current Configuration Scope: select your storeview (i select France) .then click on Locale Options Tab. From locale select Frech(France) and save store view. Do same for English and select United kingdom.Now save Config and refresh the cache of your site to see the effect.