bootstrap.php

<?php
// In config/bootstrap.php
use Cake\I18n\I18n;
use Cake\I18n\ChainMessagesLoader;
use Cake\I18n\MessagesFileLoader;

// Turn off 'default' as a fallback
I18n::translators()->useFallback(false);

// Create a custom fallback.
I18n::config('en_fallback', function ($name, $locale) {
    $loader = new MessagesFileLoader('default', 'en', 'po');
    $package = $loader();

    return $package;
});

// Redefine the 'default' package to have en_fallback as a fallback package.
I18n::config('default', function ($name, $locale) {
    // Prefer mo files over po files if they exist.
    $chain = new ChainMessagesLoader([
        new MessagesFileLoader('default', $locale, 'mo'),
        new MessagesFileLoader('default', $locale, 'po'),
    ]);
    $package = $chain();
    $package->setFallback('en_fallback');

    return $package;
});