These text strings are, precisely, known as video subtitles.
Have you ever noticed that there are text strings at the bottom of your TV screen while watching a movie? If you want to embed external subtitle, search and download subtitles right within the software and add it to the video in a streamlined process, extract subtitles from MKV, the top pick is VideoProc Converter. Summary: Here you can find top 10 subtitle editors intended for various scenarios, both online tools and standalone desktop tool included.
