I think this really hits the nail on the head. When you start out with an app, you just want to say

func: print "Hello World"

and be done with it. But when you grow, you realize that you need it to be able to print hello world in many languages so you add I18N

func: unicodePrint(geti18N(HELLO_WORLD))

getI18n(I18N_KEY): curlang.get(I18N_KEY)

1091_en: "Hello World

and then someone accidentally broke something so you want to be able to unit test it.

func(printer=stdoutprinter): printer.unicodePrint(geti18N(HELLO_WORLD))

getI18n(I18N_KEY): curlangmap.get(I18N_KEY)

1091_en: "Hello World

interface printer ...

singleton stdoutprinter ...

Ideally you'd want a framework that caters to all of these stages