Server Version#:1.29.0.6244
Player Version#: 7.0 build 23
This has been an ongoing issue for years at least. Plex for Roku is unable to display any none-latin language characters (e.g. Japanese, Chineses, etc.) on the main app UI (e.g. show/episode names, descriptions, etc.), but the subtitles work fine.
It’s likely that Roku’s system font does not support those characters, and that Plex will have to package its own font and use that instead.