Hello!
Function
public function load_textDomain()
{
load_plugin_textdomain( 'bp-better-messages', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );
}
in bp-better-messages.php calling after outputing BuddyPress nav menu.
As a solution I used this line in my plugin
//подгружаем свой языковой файл для bp-better-messages
load_plugin_textdomain( 'bp-better-messages', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );