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If you have updated your site's WordPress since September 4, 2014, the site will be running WordPress version 4 or later. Earlier versions of WordPress require a different, somewhat more complicated method described in its own section below. Also, this method is for blogs that use WordPress software, but are hosted on a third-party server. If your blog contains ".wordpress.com" in the title, refer to the wordpress.com method below instead. If you don't remember when you updated, visit (yoursitename).com/readme.html and look near the top of the page for a WordPress version number. WordPress has been translated into many languages. Each translation has a file ending in the extension ".mo". You can get these files by locating the desired language on this list, clicking "More" on the same line, then clicking "Download language pack." If you do not see a download link, the translation may be incomplete or not updated to WordPress v4. If the language pack has multiple ".mo" files, look up the language code, as well as the country code if the language is spoken in multiple countries. The file name will always follow the format languagecode.mo or languagecode_COUNTRYCODE.mo. For example, en.mo is a generic English language translation. en_GB.mo is an English translation using Great Britain regional spelling. Go to the /wp-content directory on your WordPress site's server. If there is not already a folder called /languages, create one with exactly that name. Upload the .mo file corresponding to the desired language into your /languages folder. If you have never uploaded files to your server before, you will need to use an FTP client, or the file management system provided by your hosting service. WordPress recommends FileZilla for Windows, or CyberDuck for Mac. Log into your site as an administrator. Click Settings→general→Site Language. Select the language option corresponding to the .mo file you just uploaded. The selected language should now be your site's default language.

Summary:
Use this method for WordPress version 4. Download a WordPress language file. Identify the correct file. Locate or create a /languages folder in your site directory. Upload the file to your /languages folder. Change the language in the admin settings.