Double every translation string for a given text domain for testing purpose.

double-translation-strings.php

<?php
/**
 * Plugin Name: Double every translation
 * Description: Double every translation string for testing purpose.
 * Plugin URI:  https://torstenlandsiedel.de
 * Version:     1.0
 * Author:      Torsten Landsiedel
 * Author URI:  https://torstenlandsiedel.de
 * Licence:     GPL 2
 * License URI: http://opensource.org/licenses/GPL-2.0
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly.
}

/**
 * Double every translation for testing
 *
 * @param  string $translated_text   Output of the translation function.
 * @param  string $untranslated_text Original string in en_US.
 * @param  string $domain            Textdomain.
 * @return string                    Modified translation output (double string length).
 */
function double_every_translation_length( $translated_text, $untranslated_text, $domain ) {
	if ( 'antispam-bee' === $domain ) {
		$translated_text = $translated_text . ' ' . $translated_text;
	}
	return $translated_text;
}
add_filter( 'gettext', 'double_every_translation_length', 10, 3 );