Spanish number spelling game

import random


def number_to_word(n):
    assert 0 < n < 10**9, "This only works for numbers between 1 and 999,999,999"

    irregulars = {0: "",
                  1: "uno",
                  2: "dos",
                  3: "tres",
                  4: "cuatro",
                  5: "cinco",
                  6: "seis",
                  7: "siete",
                  8: "ocho",
                  9: "nueve",
                  10: "diez",
                  11: "once",
                  12: "doce",
                  13: "trece",
                  14: "catorce",
                  15: "quince",
                  16: "dieciséis",
                  17: "diecisiete",
                  18: "dieciocho",
                  19: "diecinueve",
                  20: "veinte",
                  21: "veintiuno",
                  22: "veintidós",
                  23: "veintitrés",
                  24: "veinticuatro",
                  25: "veinticinco",
                  26: "veintiséis",
                  27: "veintisiete",
                  28: "veintiocho",
                  29: "veintinueve",
                  30: "treinta",
                  40: "cuarenta",
                  50: "cincuenta",
                  60: "sesenta",
                  70: "setenta",
                  80: "ochenta",
                  90: "noventa",
                  100: "cien",
                  200: "doscientos",
                  300: "trescientos",
                  400: "cuatrocientos",
                  500: "quinientos",
                  600: "seiscientos",
                  700: "setecientos",
                  800: "ochocientos",
                  900: "novecientos",
                  1000: "mil",
                  1000000: "un millión"
                  }

    if n in irregulars:
        return irregulars[n]

    final_string = ""

    # get a lsit of the 3 digit values seperated by commas in n
    three_digits = []
    for i in range(len(str(n)) // 3 + 1):
        if n % 1000 != 0 or str(n)[-3:] == "000":
            three_digits.append(n % 1000)
            n //= 1000

    def upto999(n):
        if n in irregulars:
            return irregulars[n]
        elif 0 < n < 100:
            return "{} y {}".format(irregulars[n - n % 10], irregulars[n % 10])
        elif str(n)[0] == "1":
            return "ciento {}".format(upto999(n % 100))
        else:
            return "{} {}".format(irregulars[n - n % 100], upto999(n % 100))

    if len(three_digits) >= 1:
        final_string = str(upto999(three_digits[0]))

    if len(three_digits) >= 2:
        if three_digits[1] == 0:
            pass
        elif three_digits[1] == 1:
            final_string = "mil " + final_string
        else:
            final_string = str(
                upto999(three_digits[1])) + " mil " + final_string

    if len(three_digits) >= 3:
        if three_digits[2] == 0:
            pass
        elif three_digits[2] == 1:
            final_string = "un millión " + final_string
        else:
            final_string = str(
                upto999(three_digits[2])) + " milliones " + final_string

    return final_string


def main():
    streak = 0
    print("\n\nNote: All numbers must be between 1 and 999,999,999 for this program to work.")
    low = int(input(
        "\nPlease input the lower bound of the numbers you want to be tested on: "))
    high = int(input(
        "\nPlease input the upper bound of the numbers you want to be tested on: "))

    play_again = True
    while play_again:
        test = random.randint(low, high)
        user_guess = input(
            "\nPlease try and spell the number {}: ".format(test))
        if user_guess.lower() == number_to_word(test):
            streak += 1
            print("\nCorrect! Your current streak is {}".format(streak))
        else:
            streak = 0
            print("\nSorry, that wasn't correct. The correct spelling was \"{}\"".format(
                number_to_word(test)))

        while True:
            what_next = input(
                "\nWhat do you want to do now? (P)lay again, (Q)uit, or (C)hange numbers? ").upper()
            if what_next == "P":
                break
            elif what_next == "Q":
                play_again = False
                break
            elif what_next == "C":
                low = int(input(
                    "\nPlease input the lower bound of the numbers you want to be tested on: "))
                high = int(input(
                    "\nPlease input the upper bound of the numbers you want to be tested on: "))
                break
            else:
                print("\nSorry, that wasn't a recognised command")


if __name__ == "__main__":
    main()