In most cases where multilingual is required, I end up using Drupal, but in this case I really needed a WordPress solution.

On WordPress.org you can find a lot of information about setting up a multilingual WordPress — http://codex.wordpress.org/Multilingual_WordPress

My initial idea was to opt for a “multi-site” solution so I started exploring Multilingual Press and Multisite Language Switcher.

Multilingual Press
http://wordpress.org/extend/plugins/multilingual-press/
I do like the workflow of this plugg-in, but the lack of support for custom types and the need to debug from the start, discouraged me to use his plugin.

Multisite Language Switcher
http://wordpress.org/extend/plugins/multisite-language-switcher/
Probably the best concept as you can link content to other content types.  But it requires a translated WordPress interface for every language (installing a .mo file) and it becomes rather cumbersome in case you have to manage a lot of content.

WPML – ($79)
http://wpml.org/
Until now the best solution, although it’s not using a multi-site environment but offers the same user experience.