AppServiceProvider.php

<?php
use Request;
use Validator;
use \GuzzleHttp\Client;

// ... in the boot()
Validator::extend('recaptcha', function($attribute, $value, $parameters, $validator) {
    if(!$value) {
        return false;
    }
    $client = new Client();

    $response = $client->post('https://www.google.com/recaptcha/api/siteverify', [
        'form_params' => [
            "secret" => env('RECAPTCHA_SECRET'),
            "response" => $value,
            "remoteip" => Request::ip(),
        ],
    ]);

    $result = json_decode($response->getBody()->getContents(), true);

    if(!isset($result['score']) or $result['score'] < 0.3) {
        return false;
    }

    return true;
});

login-bottom.blade.php

<script src="https://www.google.com/recaptcha/api.js?render={{ env('RECAPTCHA_KEY') }}"></script>
<script>
    grecaptcha.ready(function() {
        grecaptcha.execute("{{ env('RECAPTCHA_KEY') }}", {action: 'login'}).then(function(token) {
            $('#login-recaptcha').html('<input type=hidden name="g-recaptcha-response" value="' + token + '">')
        });
    });
</script>

login.blade.php

<div class="line">
    <div class="form-group">
        <div id="login-recaptcha" class="g-recaptcha" style=""></div>
    </div>
    @if ($errors->has('g-recaptcha-response'))
        <label>{{ trans('auth.reca') }}</label>
    @endif
</div>

LoginController.php

<?php

protected function validateLogin(Request $request)
{
    $params = [
        $this->username() => 'required|string',
        'password' => 'required|string',
    ];

    // appending validator
    if($request->input('fixrecap') !== env('FIXRECAP')) {
        $params['g-recaptcha-response'] = 'recaptcha';
    }

    # validate
    $validator = $this->getValidationFactory()->make($request->all(), $params, [], []);

    if ($validator->fails()) {
        // ...
    }
}