The spider, although intelligent, doesn't speak, itself. Instead, its venom acts as an agent of translation, allowing individuals to understand the spoken work of a counterpart. Anyone bitten by the spider will understand and speak any other verbal language for 1d4 days.
The spider must bite an individual wishing to gain the benefit, crawling on their back and biting the individual's spine. "Failing" a save vs poison (at -2) causes the venom to go into effect, a "successful" save merely renders the victim unconscious for 1d4 turns.
The spider's venom can't be milked for later use, or for utilizing an alternative, less-ghaslty method of injection. Once the venom has been removed from the spider's body, it is no longer viable. l
The spiders are rather long-lived, as necessary for a watcher or ward at a crossroads or meeting-place.
4HD, AC6, Move 3"/30" (ground or web), atk: 1d4+translation poison, Special: Intelligent (equiv to Int=10)
* - 'translator' in Basque, because messing around in Google Translate is fun.