Have you tried opensubtitles.org?
Once you find there (that is the most common source but not the only one) the subtitle to the film and in the language you need (assuming it is available, which for very new films takes some time until someone posts it and for unpopular or very old films might never happen) every media converter application for mac should allow to add the subtitle file to the movie. If you want. the media conversion applications can make a new film file including that subtitle but if it is just for seeing the movie with the subtitle you do not need to convert it. VLC is a very popular media player and it is free. By the way there are several subtitle formats and some media conversion applications request one of those and do not accept a different one but usually one has no problems.
Media conversion software can also add a language or change the one supplied using the audio of a copy of the same film in that different language. I have done this when a 3D film was only available say in English and I wanted to show it to people speaking another language.
Ed