Nasty way to override the core WordPress translations (PHP, gettext, JS, React)

wp-core-translation-override-fr_FR.php

<?php

// Load the translations and store them in the session

/**
 * Define the string messages to override
 */
if ( ! isset( $_SESSION['translation-override']['fr_FR'] ) ) {
	$_SESSION['translation-override']['fr_FR'] = [];
}

$_SESSION['translation-override']['fr_FR'] = array_merge(
	$_SESSION['translation-override']['fr_FR'],
	[
		// original translation => [singular translation, plural translation]
		'Display featured image' => [ 'Afficher l’image d’accroche' ],
  	]
);

wp-core-translation-override.php

<?php

// Load the translations and store them in the session
// @todo change this to the real file name
require_once get_stylesheet_directory() . '/languages/override/fr_FR.php';

/**
 * Use the gettext filters to override the translations.
 */
add_filter(
	'gettext',

	/**
	 * @param string $translation current translation
	 * @param string $text original translation
	 * @param string $domain translation domain name
	 *
	 * @return string
	 */
	function ( $translation, $text, $domain ): string {
		global $locale;

		return $_SESSION['translation-override'][ $locale ][ $text ][0] ?? $translation;
	},
	10,
	3
);

/**
 * Use the gettext filters to override the contextual translations.
 */
add_filter(
	'gettext_with_context',

	/**
	 * @param string $translation current translation
	 * @param string $text original translation
	 * @param string $context translation context
	 * @param string $domain translation domain name
	 *
	 * @return string
	 */
	function ( $translation, $text, $context, $domain ): string {
		global $locale;

		return $_SESSION['translation-override'][ $locale ][ $text ][0] ?? $translation;
	},
	10,
	4
);

/**
 * Use the gettext filters to override the plural translations.
 */
add_filter(
	'ngettext',

	/**
	 * @param string $translation current translation
	 * @param string $single original singular translation
	 * @param string $plural original plural translation
	 * @param int $number plural number
	 * @param string $domain translation domain name
	 *
	 * @return string
	 */
	function ( $translation, $single, $plural, $number, $domain ): string {
		global $locale;

		return $_SESSION['translation-override'][ $locale ][ $single ][1] ?? $translation;
	},
	10,
	5
);

/**
 * Use the gettext filters to override the plural contextual translations.
 */
add_filter(
	'ngettext_with_context',

	/**
	 * @param string $translation current translation
	 * @param string $single original singular translation
	 * @param string $plural original plural translation
	 * @param int $number plural number
	 * @param string $context translation context
	 * @param string $domain translation domain name
	 *
	 * @return string
	 */
	function ( $translation, $single, $plural, $number, $context, $domain ): string {
		global $locale;

		return $_SESSION['translation-override'][ $locale ][ $single ][1] ?? $translation;
	},
	10,
	6
);

/**
 * Use the load_script_translations filters to override the translations loaded with JS.
 */
add_filter(
	'load_script_translations',

	/**
	 * @param string $translations JSON representation of the translation file
	 * @param string $file translation file path
	 * @param string $handle script handle
	 * @param string $domain translation domain
	 *
	 * @return string
	 */
	function ( $translations, $file, $handle, $domain ): string {
		global $locale;

		$decoded = json_decode( $translations );
		if ( $decoded->locale_data->messages->{''}->lang == 'fr' ) {

			$decoded->locale_data->messages = (object) array_merge(
				(array) $decoded->locale_data->messages,
				$_SESSION['translation-override'][ $locale ] ?? [],
			);

			$translations = json_encode( $decoded );
		}

		return $translations;
	},
	10,
	4
);

This is only a proof of concept that doesn't address the following issues (at least).

• context of the translation is not taken into account (so every "Original content" string will have the same translation)
• handle for the JS integration is also not verified, so there is the same problem as with the "context"