Posted again in the TMDb issue: TV Seasons Get Details sometimes does not return translation - Talk — The Movie Database (TMDB)
Just to reiterate: this is nothing I can fix in SeriesGuide. The TMDB API sometimes returns the wrong cached data for a request.
SeriesGuide 65.0.1 (currently only available on the beta program) switched to new language codes which should have fixed this issue. Anyone on the beta program can confirm that translations remain in the desired language now?