Swap out i18n-js for react-18next

[robinheinze]

We currently use i18n-js via expo-localization

@mazenchami Has done a lot of research and library comparisons and i18n-js just doesn't have the features necessary to consider ourselves robustly setup for serious internationalization.

Features we would gain (not exhaustive):

• Pluralization
• Number, currency and date formatting
• context support (incl. gender)
• Complex string interpolation, formatting, etc.

So I'm proposing we transition to react-i18next