Standalone Laravel 8.25 validation with genuine Laravel messages

composer.json

// ...some entries
"require": {
    "illuminate/validation": "^8.25",
    "illuminate/translation": "^8.25"
}
// ...some more entries

some-php-file.php

// You have to initialize the validator in some class.
// This is up to you where. Here is just code you have to engage.

// As usual the use statements goes at the top of the class.
use Illuminate\Validation;
use Illuminate\Filesystem;
use Illuminate\Translation;

// Then you have to initialize the validator and add the default messages translation file.
// NB: $translationDir has to comprise the full path to your copy of the messages translation file
// from https://github.com/laravel/laravel/blob/8.x/resources/lang/en/validation.php
// So adjust the directory depth `4` in `dirname()` to reflect
// the actual location of the file on your server.
$translationDir = dirname(__DIR__, 4) . '/wj/laravel/lang';

$filesystem = new Filesystem\Filesystem();
$fileLoader = new Translation\FileLoader($filesystem, $translationDir);
$fileLoader->addNamespace('lang', $translationDir);
$fileLoader->load('en', 'validation', 'lang');

$translator = new Translation\Translator($fileLoader, 'en');

// After this messages are already incorporated into the validator.
$factory = new Validation\Factory($translator);

// Now the example use case
$dataToValidate = ['title' => 'Some title'];
$rules = [
    'title' => 'required',
    'body' => 'required',
];

$validator = $factory->make($dataToValidate, $rules);

// This would show the error message for missing `body` item.
if ($validator->fails()) {
    $errors = $validator->errors();
    foreach ($errors->all() as $message) {
        var_dump($message);
    }
}


// Now it is where pure enjoiment comes :))

Based on StackOverflow solution from @vivanov and this blog post from Jeff.

For details as well see my original answer on StackOverflow.