If your video file is Japanese language use a whisper model optimized for Japanese. Once it produces the Japanese srt you can get translations from open ai. Use handbrake to add the srt to the file and you’re done. Good luck!

I maintain my own media library and I ensure every file has English and German subtitles. There are a variety of ways to source srt files but when all else fails a machine with enough compute can transcribe video files using open source whisper. After I generate an English srt file from the video I send it to OpenAI to create the German translation.