I can't even format a number to currency yet!

I assumed I must be reading the doco’ wrong, but I thought certain items where already installed and available in Expo. So when I try to format a number to a currency

i18n.toPercentage(100) // fails
I18n.toPercentage(100) // fails
String.format(…) // is not a function

The doco’ on Localization left me feeling lost.

What am I missing? I only need to format a number!