I'm wanting to select a certain subtitle stream in ffmpeg and I'm not sure how to do it. I've tried using -slang, which to my knowledge no longer works. The output file uses no subs as default and I want to change it to use eng subs.

what exactly are you trying to achieve, what have you tried so far and where & how is it failing?

give us more information, post commands and output please.

Sorry for the long delay. Had to work. The problem seems to happen when I want to change the subtitle, that's all I would like to happen in the output file. The normal output for this is no subtitles at all, I just want to be able to choose which subtitle stream to use at will.
For reference here is a script I'm trying to get to work
I understand if this comes off as a rookie type of question, but I can't really find any good documentation for it as the manpages in ffmpeg have yet to be updated. For example. -slang was removed due to redundancy, yet the documentation still exists.

no, it doesn't.
except maybe for the fact that you are omiting important things.
like, how do you even know the input file has subtitles, and how many?
how is the input file encoded?

as a general courtesy (meaning: i have no clue) i'll point you to
http://ffmpeg.org/
there's forums, documentation, a wiki...

I'm using mpv to select subtitles, and I'm not sure what it would be encoded as. Also, thanks for the link.