SketchUp’s own LanguageHandler requires the folder structure. It makes sense considering that there could potentially be more resources that require localization (images, skp, html without JavaScript based translation). The majority of simple scripts may get around without further resources.
(Hm, I still don’t understand why LanguageHandler was made part of the API without a final consulting with the Ruby developers. As part of the API, it’s unlikely to ever change.)
SketchUp supports the languages that the executable is available in. The downloads each ship only one language (+English), but are otherwise mostly identical. Each language just brings its own subfolder in SketchUp ##/resources/. (There used to be also SU8 in ru and nl). With Sketchup.get_locale you can obtain the current language used by SketchUp. It can only be one of these, if your operating system is set in a different language, SketchUp’s API method only returns a locale supported by SketchUp (usually fallback to en-US).
Fun fact: With a start-up parameter -lang=fr the locale can be overriden with whatever string you like – even pfl – and it would then look for a pfl folder.