Convert numbers to word form

Example:
I'd like to convert 15 to fifteen.

This converts to the word form of whichever language your default macOS voice happens to be using.
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I think you’ll have to send your number to a scripting language that has a library for converting numbers to words. Python has the inflect module, and I’m sure Perl and Ruby have similar libraries. Which you choose depends on which language you’re most familiar with.