A very simple plugin, adds a placeholder %potioneffect% that has logic like this:
If the player has no potion effects, returns "none"
If the player has more than one potion effect, returns "You have <n> Potion Effects, use /effects to display them."
If the player has one potion effect, returns the name of it.

Also provides the command /effects, which sends the player a list of each effect, its amplifier, and its duration.