Get rid of react-native-i18n

[machour]

Once #777 is merged, react-native-i18n will become almost unused.

The only two things it's used for will be:

• Retrieve the current locale (functionality provided by react-native-device-info)
• Lookup translations strings (easily doable in t()

Getting rid of it would mean having 100% control about how our translations are handled in our code.

Opinions?

[vonovak]

I'm also migrating from react-native-i18n to js-lingui. So far so good, albeit I use the i18n.t more than <Trans> components and it took me a while to set everything up (lingui/js-lingui#176). Good luck.

[machour]

@vonovak Good choice, it's an awesome lib! Actually there was a first PR to migrate to js-lingui as well (#747), but since it was too much to handle, we went for an intermediary home-made version to ease up the final migration :)