you need to install the appropriate language pack - adding the language only adds the ability, to create a translation of static texts by your own and present you the language selector in the Skin (if included). To get the translation displayed, you need to have the language selector in your skin (see default skin) and should alter language in your user profile. If you want to create multilingual portals, you need modules for translating page properties (e.g. free EALO, you can download from Forge, see icon above) and multilingual modules. There are countless posts in this forums with additional details.