lang/i18n.go
func URLPrefix(p string) Option {
	return Option{func(o *options) {
		o.urlPrefix = p
	}}
}
func URLPrefix(p string) Option {
	return Option{func(o *options) {
		o.urlPrefix = p
	}}
}
// URLPrefix is the prefix that may be used in the url, before the language
// code itself. For example, if the prefix is '/web', the final url will be
// '/web/en'.