I have the same problem with my newly purchased Roku TV, and I can't believe Roku do not support other encodings subtitles! There are thousands of non-English speakers living in the regions such as US and Canada that Roku sell its products. Arabic, Persian and Hebrew native speakers are totally being ignored by Roku. I read that Japanese and Koreans actually all non-Latin languages have the same problem.
People are asking Roku for adding other encoding support even since 2013! I had much older TVs in Tizen that was able to show persian and Arabic subtitles easily!