You can change the subtitle format on TV. While playing the movie in the bottom panel (where the play / pause buttons etc.) scroll to the right, additional options will be displayed, including SUB. There you can change the formatting to Lat2 or Lat1 (besides the color of the subtitles, font size), sometimes Lat2 works for me and sometimes Lat1.
The problem with WebOS is that it always loads the subtitles in the default format. When downloading subtitles via Napi Projekt, I tested various encodings (UTF-8, ANSI, etc.) and the TV never automatically loaded subtitles in the correct format, each time I change the format manually on the TV to Lat2 (other settings such as color and font size are remembered by me).
From the LG technical support, I also got a test file of Polish characters, of course the test TV failed. Loaded the subtitles in the default format, manually changing the encoding helped.