Small Kotlin helper function for converting a number to its textual representation, with a heavily functional approach.

IntegerToText.kt

fun main() {
    while (true) {
        val number = input("Number: ").toIntOrNull()
        if (number != null)
            println(number.getName())
        else
            println("Invalid Number!")
    }
}

fun input(message: String): String {
    print(message)
    return readln()
}

val units = listOf("zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine")
val specialTens =
    listOf("ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen")
val tens = listOf("twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety")
val orders = listOf("thousand", "million", "billion", "trillion", "quadrillion", "quintillion", "sextillion", "septillion", "octillion")

fun Int.getName(): String = when (this) {
    in 0..9 -> units[this]
    in 10..19 -> specialTens[this]
    20, 30, 40, 50, 60, 70, 80, 90 -> tens[this[0] - 2]
    in 21..99 -> "${tens[this[0] - 2]} ${units[this[1]]}"
    in 100..999 -> buildString {
        append("${units[this@getName[0]]} hundred")
        if (this@getName % 100 != 0)
            append(" ${this@getName[1..2].getName()}")
    }
    else -> buildString {
        val length = this@getName.length
        val order = (length - 1) / 3 // 1 for thousands, 2 for millions, etc...
        append("${this@getName[0..length - (3 * order + 1)].getName()} ${orders[order - 1]}")
        if (this@getName % orderDivider(order) != 0)
            append(" ${this@getName[length - 3 * order until length].getName()}")
    }
}

operator fun Int.get(index: Int) = "$this"[index].digitToInt()

operator fun Int.get(range: IntRange) = "$this".substring(range).toInt()

fun orderDivider(order: Int) = "1${"0".repeat(3 * order)}".toInt()

val Int.length
    get() = "$this".length

For now this is limited by Int's constraints, but may be improved by using Long or BigInt instead.