[9.x] Add auto language fallback to less specific localization

[emiliopedrollo]

Hi everyone,

A bit of context first: as my team is preparing to undertake a huge effort on localization we've been looking for specialized software and personal online and we figured out that the vast majority charge either by word or "target translation key" (as key x language).

Here's our problem: we plan to support locales like en_US and en_GB that would have most of the translations keys exactly the same, having those fallback to en could reduce drastically the number of strings in the end. As this is currently possible using the current fallback config when we add pt_BR and pt_PT to the mix suddenly there's no way to have those fallback to pt.

This proposal adds auto fallback when the locale is something like en_US or en-US to a less specific en. It could also fallback something like en_US-custom to en_US then en.

As a downside, I realize that this could add several CPU cycles to process the regular expression and even worse, a lot more of I/O.

[taylorotwell]

Instead, can you PR a method like determineLocalesUsing(fn) that accepts a Closure. That way you can invoke that from your own service provider and do whatever custom behavior you wish. Thanks!