put this in any random file into /wp-content/mu-plugins/ to get your custom language files back in bbPress

functions.php

<?php
// Exit if accessed directly
defined( 'ABSPATH' ) || exit;

function fix_bbpress_custom_language_packs( $bbpress = null ) {
	// No need to perform this action, it's too late!
	remove_action( 'bbp_load_textdomain', array( $bbpress, 'load_textdomain' ), 5 );

	/**
	 * To be sure the custom language pack is loaded, let's be the first to load the textdomain
	 * 
	 * NB: bbp_loaded is hooking 'plugins_loaded' at 10. Hooking 'plugins_loaded' later than 10
	 * won't load the custom language pack
	 */ 
	add_action( 'bbp_loaded', array( $bbpress, 'load_textdomain' ), 0 );
}
add_action( 'bbp_after_setup_actions', 'fix_bbpress_custom_language_packs', 10, 1 );