Curse of tongues allows the caster to steal the target's voice with a curse, replacing it with the voice of a terrible demon. The subject's new voice only speaks in the Eredar tongue, regardless of any other spells or effects on the target. This spell prevents the target from using any spell with verbal components. 
Cast Time Inc
Targets affected by this spell will actually speak in Demonic in emotes and in chat. A CM on Blizzard's official forums joked about how coders spent time coding this curse to do just that.
Suppression reduces the chance for the curse to be resisted; Grim Reach increases the range; and Contagion reduces the chance that it'll be dispelled.
Tips and tactics
CoT is used primarily against casters to limit their spellcasting ability.
Mages can remove curses and the talented ones will do so.
Patch 3.0.9 (2009-02-10): Now increases the casting time of all spells by 25% (Rank 1) and 30% (Rank 2), down from 50% and 60%.
Patch 2.1.0 (22-May-2007): Now has a 12 second duration when used on PvP targets.