Subtitles for most movies are really easy to find online. I use subtitlist.net for this, and Subtitle Edit for obscure items in my collection that don't have publicly available subtitles online.
If you put the SRT files in the same directory as your items, with the same name, PLEX should pull it and use it as the subtitle for the movie or episode it belongs to.
null@slrpnk.net 10 months ago
Plex can fetch subtitles for media where you didn’t include them.