app.php

<?
// config/app.php

'App' => [
        'defaultLocale' => env('APP_DEFAULT_LOCALE', 'es'),
        // ...
        ],
    ],

bootstrap.php

//config/bootstrap.php
ini_set('intl.default_locale', 'es');

default.po

#Locale/es/default.po
#: Controller/UsersController.php:52;77
msgid "The user has been saved."
msgstr "El usuario ha sido creado"

#: Controller/UsersController.php:55;80
msgid "The user could not be saved. Please, try again."
msgstr "El usuario no ha sido creado. Por favor, inténtelo de nuevo"

#: Controller/UsersController.php:99
msgid "The user has been deleted."
msgstr "El usuario ha sido eliminado"

#: Controller/UsersController.php:101
msgid "The user could not be deleted. Please, try again."
msgstr "El usuario no se ha eliminado. Por favor inténtelo de nuevo"

#: Template/Element/carousel.ctp:15;38
msgid "Hello!"
msgstr "Hola"

#: Template/Element/carousel.ctp:16;39
msgid "I'm available for work."
msgstr "Estoy lista para trabajar"

#: Template/Element/carousel.ctp:25
msgid "Available for:"
msgstr "Disponible para"

#: Template/Element/carousel.ctp:27
msgid "Quality Control"
msgstr "Control de Calidad"

#: Template/Element/carousel.ctp:28
msgid "Process Analyst"
msgstr "Analista de Procesos OyM"

#: Template/Element/footer.ctp:9
msgid "Menu"
msgstr "Menú"

#: Template/Element/footer.ctp:17
msgid "Gallery"
msgstr "Galería"

#: Template/Element/footer.ctp:30
msgid "Have a question?"
msgstr "¿Preguntas?"

#: Template/Element/footer.ctp:34
msgid "Email address"
msgstr "Correo Electrónico"

#: Template/Element/footer.ctp:42
msgid "Get in touch!"
msgstr "Mantén el contacto"

#: Template/Element/footer.ctp:44
msgid "Industrial Engineer for Hire"
msgstr "Ingeniero industrial para Contrartar"

footer.ctp

<!-- src/Template/Element/footer.ctp -->
<!-- Element also appears on Home page -->

<h3><?= __("Have a question?") ?></h3>

PagesController.php

<?
//Controller/PagesController.php
// at the top
use Cake\I18n\I18n;

//inside a view method, homepage

public function home()
{
    I18n::locale('es');
}