Switching between translations

I may have forgot something, but it doesn't seem that it is possible to switch from 1 translation to another using the API.

Am I wrong ?

I think to achieve this, there should be a `pickadate.defaults.langs[localeCode]` allowing to override some `pickadate.defaults` when changing a lang

Something a bit like [what momentjs is doing](https://github.com/moment/moment/blob/develop/locale/fr.js#L12) with its `defineLocale()` call

WDYT ?