Hi! I started having this annoying issue on my 42" 42LA740S after an update, probably the same as you. Out of the box it was possibile to switch language and subtitles while streaming (I use Plex as media server on my PC). I'm trying to get an answer from the LG italian support, but I expect nothing
I think they removed this functionality from the DLNA player for some reason, and they are never putting it back Maybe if we flood the support with emails.... edit: I found this article explaining how to downgrade TV firmware http://blog.techflaws.org/2014/05/26/firmware-downgrade-for-lg-smart-tvs/
I'm not new to this kind of thing, but I think is a little dangerous because there is no "recovery mode" or a way to flash the firmware directly from a pendrive USB, but you need to trick the TV to make it download a specific version of the software from your PC instead of LG official website.