Panasonic Viera TVs only support playing external subtitles with DivX (AVI) files at the moment, so you’ll need to convert your MKV to an AVI (mpeg4 video with mp3 or ac3 audio) for subtitles to work. Hopefully, Panasonic will improve their DLNA firmware to support other formats, but at the moment you are limited to DiVX files if you want external subtitles to display.