WPML is a plugin/pluginsuite for the WordPress CMS. WPML is actually an abbreviation of WordPress MultiLingual (plugin).
WPML is the most popular plugin for websites that use WordPress as their CMS and are multilingual. For WPML it does not matter much if you have 2 languagues available or 5. The process is the same. With WPML you can manage the different languages on the same domain with one single WordPress installation (instead of having multiple installations)
It works with almost any theme or plugin, and if there is a problem there is always the support department to help you out. You can translate the content yourself or have an external agency do the translations and you can also choose whether you want all posts and pages translated or not (and how WPML should handle these: you might want WordPress to show the original content if no translation is available, of just have that content item not available in that language).