Hi guys, author here, as far as I have tested it is working with Chrome and Firefox in desktop (where you can drag and drop)
It really depends on if the subtitle is in the Open Subtitles API or not, it tries to search for the best suitable subtitle there.
I've only tested with spanish and english languages but I am planning on improving the localization of the app.
Thanks for the feedback and glad some of you are finding use in it.