That is easy. Press the OK button once, you should get playback controls. Pause it, then go right until you reach the audio and the subtitle setting. You should go up and then enable the subtitles. One other way is to set it up un Settings and selecting the preferred language of the subtitle.
The language of the subtitle is given in its name. Considering you would have a movie called Shrek.mkv and a subtitle called Shrek.srt, by renaming it to Shrek.en.srt, Kodi or any other player will know the language of the subtitle. For instance, a Romanian subtitle would be named Shrek.ro.srt. This is how I organized them for my rips. I usually have an English and a Romanian subtitle for each of them.