The standard does not specify what the standard exceptions return from what(). It does not have to be in English. Why care about it's prototype? You may want t

The standard does not specify what the standard exceptions return from what(). It does not have to be in English. Why care about it's prototype? You may want to embed into what() unicode strings that describe the error and came from elsewhere. E.g. a path, a URL, an XML element id, etc. from the context the exception originated. It may be shown to the user or written to the log. Localization is irrelevant here.