Fix for multi-country locales supported by Translatable

[antonioribeiro]

Description

If we install Laravel and Twill and don't configure Translatable right away, this error will happen:

Because Twill doesn't support multi-countries locales present on Translatable:

'locales' => [
    'en',
    'fr',
    'es' => [
        'MX', // mexican spanish
         'CO', // colombian spanish
    ],
],

This PR basically fix this by changing the i18_helpers->getLocales() to support them and return:

[
    "en",
    "fr",
    "es-MX",
    "es-CO",
]

It also replaces some instances of config('translatable.locales') by the helper.

[ifox]

Hi @antonioribeiro, that's great to support it! For context, Twill publishes its own version of the translatable.php config file with only 'en' in the locales by default. Did you publish from the translatable package itself? Still I'm going to test this and merge it!