WPML makes it easy to build multilingual sites and run them. It’s powerful enough for corporate sites, yet simple for blogs.
WPML – The WordPress Multilingual Plugin
WPML is the localization solution I use myself on several multilingual websites. It is very complete, sticks to WordPress localization standards, and it supports and is supported by a wide range of other plugins. It is a powerful multilingual solution that offers a lot of additional features, including a pretty satisfying auto translation feature, so I recommend it vividly.