Note: this article is about displaying the WordPress Administrative "back-end" in your language. If you are looking for information on how to localize your "front-end" website, or customize your theme to be localizable, refer to i18n for WordPress Developers (and optionally  and  for theme developers. If you are interested in how to build a multilingual (e.g.: French / English) WordPress site, you can start your journey here.
Although WordPress displays in U.S. English by default, it has the built-in capability to be used in any language. The WordPress community has already translated WordPress into many languages, and there are Themes, translation files, and support available in many other languages (see WordPress in Your Language).
As of version 4.0, you can have WordPress automatically install the language of your choice during the installation process.
For WordPress 4.1 or later, you can install language packs directly from the Admin back-end at any time. WordPress will download them and switch the admin back-end to that language. Navigate to Settings > General > Site Language and select from the list of available languages. For Multisite Super Admins, you can set the default language using the Network Administration Settings pane.
Here are the steps you will need to follow to install an international version of WordPress.
Note: If you make an error in the steps or you do not specify the correct language, WordPress will default back to English. For more help Installing WordPress, see Installing WordPress and FAQ Installation.
You can use the plugin wp-native-dashboard to automatically download language files stored in the wordpress svn repository. However, as of the time of this writing, the plugin is 2 years out of date and only supports a very small selection of languages.
define ('WPLANG', '');
define ('WPLANG', 'pt_BR');
define ('WPLANG', 'en_GB');
If you have a site network (WordPress multisite), the language is set on a per-blog basis through the "Site language" option in the Settings > General subpanel.
You can set the default language for the entire network under the Network Admin > Settings panel ("Default Language").
If you want to customize the translations displayed on your screen, or add translations for terms that are still displaying in English after installation, you will need to download the .po file corresponding to the .mo language file you installed. Then you will need to translate additional terms and create a new .mo file -- see Translating WordPress for more information.
You may also need to translate a few other files -- see Files For Direct Translation for more information.