Translate WooCommerce and WooCommerce Germanized mails into order language with TranslatePress before sending. Note: As of TranslatePress Version 2.3.5, this functionality was implemented natively by TranslatePress, making this Code obsolete. Read more here: https://translatepress.com/docs/translating-woocommerce-emails/

functions.php

// Save the current language in post_meta when checkout is processed (used to identify correct Email language)
add_action('woocommerce_checkout_update_order_meta', 'dartrax_save_language_on_checkout', 10, 2 );
function dartrax_save_language_on_checkout( $order_id, $posted ) {
	if( ! class_exists('TRP_Translate_Press') ) return '';
	global $TRP_LANGUAGE;
    update_post_meta( $order_id, 'order_language', $TRP_LANGUAGE );
}

// Woocommerce Germanized Mails
add_action( 'woocommerce_gzd_shipment_status_draft_to_shipped_notification', 'dartrax_prepare_locale_for_Mail_with_shipment_id', 5, 1 );
add_action( 'woocommerce_gzd_shipment_status_processing_to_shipped_notification', 'dartrax_prepare_locale_for_Mail_with_shipment_id', 5, 1 );
add_action( 'woocommerce_gzd_return_shipment_status_draft_to_processing_notification', 'dartrax_prepare_locale_for_Mail_with_shipment_id', 5, 1 );
add_action( 'woocommerce_gzd_return_shipment_status_requested_to_processing_notification', 'dartrax_prepare_locale_for_Mail_with_shipment_id', 5, 1 );
add_action( 'woocommerce_gzd_return_shipment_status_processing_to_delivered_notification', 'dartrax_prepare_locale_for_Mail_with_shipment_id', 5, 1 );
add_action( 'woocommerce_gzd_return_shipment_status_shipped_to_delivered_notification', 'dartrax_prepare_locale_for_Mail_with_shipment_id', 5, 1 );
function dartrax_prepare_locale_for_Mail_with_shipment_id( $shipment_id ) {
	// get order_id from shipment and proceed with that
	if( $shipment = wc_gzd_get_shipment( $shipment_id ) )
		dartrax_prepare_locale_for_Mail_with_order_id( $shipment->get_order_id() );
}

// Woocommerce Shipment Mails
add_action( 'woocommerce_order_status_processing_to_cancelled_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_on-hold_to_cancelled_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_completed_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_pending_to_on-hold_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_failed_to_on-hold_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_cancelled_to_on-hold_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_cancelled_to_processing_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_failed_to_processing_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_on-hold_to_processing_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_pending_to_processing_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_fully_refunded_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_partially_refunded_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_pending_to_failed_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_on-hold_to_failed_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_pending_to_completed_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_failed_to_completed_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
add_action( 'woocommerce_order_status_cancelled_to_completed_notification','dartrax_prepare_locale_for_Mail_with_order_id', 5, 1 );
function dartrax_prepare_locale_for_Mail_with_order_id( $order_id ) {
	// Get language code that may be stored in post meta data
	$lang_code = get_post_meta($order_id, 'order_language', true);
	if( $lang_code == '' || ! class_exists('TRP_Translate_Press') ) return '';
	// Set it as current active language for translatepress
	global $TRP_LANGUAGE;
	$TRP_LANGUAGE = $lang_code;
}

// Woocommerce Mails when resend by Admin
add_action( 'woocommerce_before_resend_order_emails','dartrax_prepare_locale_for_resend_Mails', 5, 2 );
function dartrax_prepare_locale_for_resend_Mails( $order, $mail_type ) {
	// First type belongs to WooCommerce, last two types belong to WooCommerce Germanized
	if($mail_type == 'customer_invoice' || $mail_type == 'customer_paid_for_order' || $mail_type == 'customer_processing_order')
		dartrax_prepare_locale_for_Mail_with_order_id( $order->get_id() );
}

// Woocommerce Note to customer Mail
add_action( 'woocommerce_new_customer_note_notification','dartrax_prepare_locale_for_note_Mails', 5, 1 );
function dartrax_prepare_locale_for_note_Mails( $note_and_order_id ) {
	// get order_id from argument array and proceed with that
	dartrax_prepare_locale_for_Mail_with_order_id( $note_and_order_id['order_id'] );
}

// Prevent Woocommerce Germanized from filtering the Translatepress Shortcut when legal pages are used in emails
add_filter( 'woocommerce_gzd_email_attachment_content_shortcodes_allowed', function( $shortcodes_allowed ) {
	array_push($shortcodes_allowed, "trp_language");
	return $shortcodes_allowed;
} );

// Override Locale when WooCommerce sends an eMail
add_filter( 'woocommerce_email_setup_locale', function() {
	if( ! class_exists('TRP_Translate_Press') ) return '';
	// Override translatepress 'locale'-function because that does not work in Admin interface
	add_filter('locale','dartrax_force_trp_locale', 99999 + 1);
	// Switch text domains to load the correct .po/.mo-file based translations
	global $TRP_LANGUAGE;
	switch_text_domains( $TRP_LANGUAGE );
	return false;
} );
add_filter( 'woocommerce_email_restore_locale', function() {
	// Undo overriding of translatepress' 'locale'-function
	remove_filter('locale','dartrax_force_trp_locale', 99999 + 1);
	return true;
} );

// Override translatepress 'locale'-function because that does not deliver $TRP_LANGUAGE in Admin interface
function dartrax_force_trp_locale($locale) {
	global $TRP_LANGUAGE;
	return $TRP_LANGUAGE;
}	

// Override 'plugin_locale'-function so Woocommerce and Woocommerze Germanized won't use the admin profile language
function dartrax_force_woo_locale($locale, $plugin) {
	global $TRP_LANGUAGE;
	return $plugin == 'woocommerce' || $plugin == 'woocommerce-germanized' ? $TRP_LANGUAGE : $locale;
}

// Switch to another text domain. Inspired by https://gist.github.com/Jon007/5b90e78289899bc28e9c39c12ef56e60
function switch_text_domains( $language ) {
	if ( class_exists( 'TRP_Translate_Press' ) && class_exists( 'WooCommerce' ) && class_exists( 'WooCommerce_Germanized' ) ) {
		global $locale, $woocommerce, $woocommerce_germanized;
		// unload plugin's textdomains
		unload_textdomain( 'default' );
		unload_textdomain( 'woocommerce' );
		unload_textdomain( 'woocommerce-germanized' );
		// set locale to order locale
		$locale	= apply_filters( 'locale', $language );
		// Woocommerce and Woocommerce Germanized use the admin profile language instead of the side language. Override with the desired language
		add_filter('plugin_locale', 'dartrax_force_woo_locale', 10, 2);
		// (re-)load plugin's textdomain with order locale
		load_default_textdomain( $language );
		$woocommerce->load_plugin_textdomain();
		$woocommerce_germanized->load_plugin_textdomain();
		$wp_locale = new \WP_Locale();
		// Clean up
		remove_filter('plugin_locale', 'dartrax_force_woo_locale', 10);		
	}
}

Fix: Since a recent update of WooCommerce Germanized, the trp_language conditional shortcode got ignored when used in legal pages included with mails. This resulted in mails in which, for example, the terms and conditions occurred several times in a row in all available languages. This is fixed now.

For everyone's interest:
I've used the new Mail localisation feature that was natively build into TranslatePress for a while now, and I did not encounter any problems.

Just this little peace of code is still necessary, if you want to use the TranslatePress' conditional shortcuts in Emails, because otherwise they get filtered by the Germanized Extension.

// Prevent Woocommerce Germanized from filtering the Translatepress Shortcut when legal pages are used in emails
add_filter( 'woocommerce_gzd_email_attachment_content_shortcodes_allowed', function( $shortcodes_allowed ) {
	array_push($shortcodes_allowed, "trp_language");
	return $shortcodes_allowed;
} );