You really ought to have done a search on this. You'll have Rene getting his guns out and cleaning them
The answer is: not much. Compared with an audio track, say, the removal or retention of a single subtitle will hardly impact on your movie's quality at all.
If there are lots of them and you will only ever want English (or whatever your mother tongue may be), then by all means delete them. Their cumulative size may be slightly significant.
Be very wary about deleting any English subtitles; sometimes they are required.