Changing the language of your WordPress website takes only a few minutes. You don’t have to do a reinstall or even to upgrade any files. In fact, the core files remain the way they are.
Changing the Language Manually
Here’s a concise step-by-step guide on changing the language of an existing WordPress site.
define ('WPLANG', '');
define ('WPLANG', 'es_ES');
Other Ways to Change the Language
The method described above is the manual way of changing your WordPress language. In many cases, changing the language can be even simpler than that because there is a good chance there is an official WordPress version out there in your own native language. For example, if you want Spanish localization, you can just grab the Spanish version of WordPress here.
For Bilingual or Multilingual Blogs
If you want to create bilingual posts or change languages between posts, there are a variety of plugins that can help you with that. Some examples:
Using WordPress in another language is easy.