[11.x] Add derived fallback locale support

[olivermbs]

This PR adds support for derived fallback locales when setting the application locale. For example, when using 'de_CH', you can optionally have it automatically determine 'de' as a fallback locale.

Added a new $useDerivedFallback parameter to setLocale():

public function setLocale($locale, $useDerivedFallback = false)
{
    $this['config']->set('app.locale', $locale);

    $this['translator']->setLocale($locale);

    $this['events']->dispatch(new LocaleUpdated($locale));

    if ($useDerivedFallback) {
        $primaryLanguage = substr($locale, 0, 2);

        if ($primaryLanguage !== $locale) {
            $this->setFallbackLocale($primaryLanguage);
        }
    }
}

[shaedrich]

Keep in mind, that locales can not only be de and de_CH, they can also have more segments.

It also raises the question if the passed in locale should be validated

public function setLocale($locale)
{
    if (!ctype_lower(substr($locale, 0, 2)) || (strlen($locale) > 2 && substr($locale, 2, 1) !== '_') || (strlen($locale) > 3 && !ctype_upper(substr($locale, 3, 2)))) {
        throw new InvalidArgumentException('Not a valid locale');
    }

which then would open the opportunity to

• replace - with _
• change case of the segments to allow for case insensitive processing later on

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!