Why is 說 translated as "pleasant" in Confucius's Analects?
說 is the original 悅, they mean the same thing. It's only because in different times, different character may be used to represent the same thing. This situation of different character used is called 古今字. Check this(Chinese Wikipedia).

To back this up, consult (清)段玉裁《說文解字注》


  説釋也。説釋卽悦懌。説悦釋懌皆古今字。許書無悦懌二字也。説釋者，開解之意。故爲喜悦。釆部曰。釋，解也。从言。兑聲。儿部曰。兑，説也。本周易。此從言兑會意。兑亦聲。弋雪切。十五部。一曰談説。此本無二義二音。疑後增此四字。别音爲失䑔切。


So as to why it's 說, it's because when you explain something (說釋->說明解釋) to someone, he or she will be happy because the you resolve (開解) their question.

Master Sparkles, I don't think call it stand-in word is appropriate since it's just the correct usage at that time. (Sorry I can't comment so I have to reply here)