but whenever I access other modules, for example "post" module, it give an error, cannot find the required language for user model (I place the model_user_m_lang.php in modules/user/language/english folder).
My MY_Lang already extends MX_Lang from HMVC without any addition.
What I think that may be a problem is lang() method in MX_Lang. It uses only active modules to get additional language folder: