It is easy to add a 'en' to a phone with which 'zs','zt' already installed, maybe your phone was flashed with XXX_CHINA_CH_RED53.mbn. but it hard to add 'zs' or 'zt' language to your phone, which flashed with XXX_GENERIC_RED53.mbn, chinese characters was display as block in it.

and for display chinese characters (for example) correctly, you should flash you phone with W710_R1JC002_MAIN_CHINA_CH_RED52.mbn and W710_R1JC002_FS_CHINA_RED52.fbn and a correct customize pack file.

/Jack