If the movie is called Avengers.mkv, I am using Avengers.en.srt for the subtitle in English and so on. The country codes are available somewhere. You can instruct Kodi what language it should prefer and the order. I am always downloading subtitles myself. This is also working on MPC Home Cinema on Windows or any other player (VLC). You set it once, keep this naming convention and you should not have any issues. It took me a while to discover this, it is not recommended anywhere and it is too logical to not be uses. This page describes the language codes pretty well https://www.science.co.il/language/Codes.php.
Edit: I think you are downloading subtitles through Kodi. Getting subtitles through Kodi is a pain the behind, that is why I am getting the English ones from opensubtitles.org and the Romanian ones from a few local websites. There are a lot of topics on this matter around here.