Module:Spellnum per MOS

 local p = {}
 local words = {"thousand", "million", "billion", "trillion"} -- We don't need to go higher than this, no-one knows what an octillion is.

 -- For use by other scripts. Takes arguments:
 -- - 1: string or number, value to convert
 -- - forcenum: string for Template:Yesno, forces a result in digits for all n ≥ 10.
 -- - formating options for spellnum: zero, adj, ord, us
 function p.spellnum(args)
	local frame = mw.getCurrentFrame()
	local numeral = tonumber((string.gsub(args[1], ",", "")))
	
 local pass_zero = "zero"
 if args['zero'] ~= nil and args['zero'] ~= '' then
 	pass_zero = args['zero']
 end
 
	-- Always return numerals for negative numbers, non-integers, and if (forcenum and numeral >= 10).
	if numeral < 0 or
			math.fmod(numeral, 1) ~= 0 or
			(numeral >= 10 and frame:expandTemplate{ title = 'yesno', args = {args['forcenum']} } == 'yes') then
		return mw.language.getContentLanguage():formatNum(numeral)
	end

	-- Convert numeral to words
	local spelled = frame:expandTemplate{ title = 'spellnum', args = {
		numeral, zero = pass_zero, adj = args['adj'], ord = args['ord'], us = args['us']}}
	
	-- Return numerals if more than two words would be needed, else return words
	if mw.ustring.find(spelled,'%a+[ %-]%a+[ %-]%a+') then
		-- Handle numbers larger than one million
		if numeral >= 1000000 and numeral <= 1000000000000000 then
			local size = math.min(4, math.floor(math.log10(numeral) / 3))
			numeral = numeral / 1000^size
			return ({"%.1f ", "%d ", "%d "})[1 + math.floor(math.log10(numeral))]:format(numeral) .. words[size]
		end
		return mw.language.getContentLanguage():formatNum(numeral)
	else 
		return spelled
	end
 end

 function p.main(frame)
	return p.spellnum(frame.args)
 end

 return p