The Doctor will take these one at a time. Display is mainly an issue with fonts. Even when you use a Unicode-enabled product such as Word 2000, you will not be able to see text in some languages unless there is a font available to support the text. When this happens, you will only be able to store the information. Input of text does get better every version, though obviously that is no consolation if the language you read and write does not have an associated keyboard yet.
Regional options are in most cases a simple convenience.
By choosing one of the locales listed in the regional options dialog either at setup time or afterwards in the control panel applet, all of the settings in the entire Regional Options dialog can be changed. You can override any of these settings that you like, which is why these options are mainly there as a convenience. By specifying the means by which an application can determine how to compare two strings, issues as simple as sorted lists and as complex as database indexes can be handled. Although many sort orders are added with each version, it would likely be impossible for Windows to support every possible sort for every dialect and language. Custom usage is when applications and components use LCIDs to help with their own unique problems and issues with changing behavior based on locale.