Python converter: number-to-English - Project Euler 17
Here's one using modulo % and list joining that uses your original NUMBER_WORDS dict:

def int_to_english(n):
    english_parts = []
    ones = n % 10
    tens = n % 100
    hundreds = math.floor(n / 100) % 10
    thousands = math.floor(n / 1000)

    if thousands:
        english_parts.append(int_to_english(thousands))
        english_parts.append('thousand')
        if not hundreds and tens:
            english_parts.append('and')
    if hundreds:
        english_parts.append(NUMBER_WORDS[hundreds])
        english_parts.append('hundred')
        if tens:
            english_parts.append('and')
    if tens:
        if tens &lt; 20 or ones == 0:
            english_parts.append(NUMBER_WORDS[tens])
        else:
            english_parts.append(NUMBER_WORDS[tens - ones])
            english_parts.append(NUMBER_WORDS[ones])
    return ' '.join(english_parts)


It works up to 999,999, but could be extended further with a little customisation.