LookMovie doesn't add subtitles, there are some automatically there, but if there is none, or the ones there are bad, the only option is to search for the subtitle file yourself on the internet and upload it into the movie yourself. These are some of the sites that I have used to find the SRT file myself before:
https://www.srtfiles.com/
http://91.234.99.182:8080/user/s666a
https://www.opensubtitles.org/en/login/vrf-on
Quite honestly, it's probably best to just do a google search "EO Spanish Subtitle file" or "EO Spanish SRT file". I don't know why, but subtitle files are notoriously often out of sync (the text doesn't happen at the same time as the speaking), or the subtitles don't translate every different language spoken in the movie (for example it may translate all the English but when someone speaks Germans it just says "Speaking German"), so it usually takes me 2 or 3 times before I find one that's perfect.
Once you have the file, click the [CC] in the lookmovie player and then click "Upload" in the top left.