I'm not really sure how you mean that you solve it today with two languages and why you would need more directories, but all text are externalized to properties-files. So to add languages you would simply add more properties-files with extensions like .get_xx_XX.properties, where xx and XX are language and country codes (for instance en and US for the english files).
The language being used is determined by the browser settings (which means that in most cases the native language of the user would be selected if its present).
Edit: If you want to develop own webscripts and use multiple languages you could look at any of the standard webscripts to see how the strings are externalized and how the properties files are used.
