ClassicPress is already used in many different countries where English is not the main language; and even where it is, there can be variations in the spelling of words. ClassicPress itself is internationalized and can be localized.
Plugins and themes developed for ClassicPress should also be internationalized so that they can then be localized. The developer who authors the plugin doesn’t have to do the localization (translations), but by internationalizing the plugin it means that others can easily translate the plugin without needing to modify any code.
By making translations easy to do, you increase the audience for your plugin as site admins can easily translate it into their sites language.