What is the best way to add i18n to an existing custom CMS system?

[mschnack]

I am currently responsible for an existing CMS, built a few years ago with C# .Net and a few clients are requesting to have their website in Spanish. The websites are pre-populated with pages from a library and currently the application has no support for i18n. What is the quickest way to allow the client to have his website in Spanish? And what is the best way to allow any language?

I understand those are two different answers, but I want to plan for future and at the same time provide a quick solution to please the clients.