Set a custom PMPro Email subject per locale.

pmpro-email-change-subject-based-on-locale.php

<?php
/**
 * Change the email subject based on the user's locale.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function pmpro_email_change_subject_based_on_locale( $subject, $email ) {

	// Set your localized subjects for each email template here.
	$localized_subjects = array(
		// French
		'fr_FR' => array(
			'checkout_check'     => "Votre sujet d'email personnalisé ici",
			'checkout_express'   => "Votre sujet d'email personnalisé ici",
			'checkout_free'      => "Votre sujet d'email personnalisé ici",
			'checkout_freetrial' => "Votre sujet d'email personnalisé ici",
			'checkout_paid'      => "Votre sujet d'email personnalisé ici",
			'checkout_trial'     => "Votre sujet d'email personnalisé ici",
		),
		// German
		'de_DE' => array(
			'checkout_check'     => 'Ihr personalisierter E-Mail-Betreff hier',
			'checkout_express'   => 'Ihr personalisierter E-Mail-Betreff hier',
			'checkout_free'      => 'Ihr personalisierter E-Mail-Betreff hier',
			'checkout_freetrial' => 'Ihr personalisierter E-Mail-Betreff hier',
			'checkout_paid'      => 'Ihr personalisierter E-Mail-Betreff hier',
			'checkout_trial'     => 'Ihr personalisierter E-Mail-Betreff hier',
		),
	);

	// Get the user's locale and the email template.
	$user        = get_user_by( 'email', $email->email );
	$user_locale = get_user_locale( $user );
	$template    = $email->template;

	// Let's only do this if the user's locale is not the default "en_US" locale.
	if ( strpos( 'en_US', $user_locale ) !== false ) {
		return $subject;
	}

	// If the user's locale is in our localized subjects array, and the email template is in the localized subjects array, set the localized subject.
	if ( array_key_exists( $user_locale, $localized_subjects ) && array_key_exists( $template, $localized_subjects[ $user_locale ] ) ) {
		$subject = $localized_subjects[ $user_locale ][ $template ];
	}

	return $subject;
}
add_filter( 'pmpro_email_subject', 'pmpro_email_change_subject_based_on_locale', 20, 2 );