Use enum case's name

enum.swift

import Foundation
extension Encodable {
    func asDictionary(convertToSnakeCase: Bool = false) throws -> [String: Any] {
        let encoder = JSONEncoder()
        if convertToSnakeCase { encoder.keyEncodingStrategy = .convertToSnakeCase }
        let data = try encoder.encode(self)
        guard let dictionary = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? [String: Any] else {
            throw NSError()
        }
        return dictionary
    }
}

enum Region: Int, Codable, CaseIterable {
    case 北海道
    case 本州
    case 中国四国
    case 九州
    
    var toString: String {
        return String(describing: self)
    }
}

struct SampleRequest: Encodable {
    let regionType: Region
}

let indexPathRowSample = Region.allCases.count - 1
print(Region.allCases[indexPathRowSample].toString)

let region: Region = .九州
let request = SampleRequest(regionType: region)
let parameters = try request.asDictionary(convertToSnakeCase: true)
print(parameters)