What language do Spirits speak? Can a garden variety summoned Spirit "speak" at all? Are their linguistic abilities related in some way to the native tongue of thier conjurer? Is there old canon on this? Is there something I'm missing in the SR4 book?

I don't ever remember it coming up in SR games i've played in and GMed. Ever.

hmm, this reminds me of a old story where someone summoned a watcher to act as a translater between himself and a person that could only speak spanish or something like that (i cant be botherd to find the old thread).

i think the GM playing as if the spirit could only talk the languages of the summoner

It would depend on spirit type, but as a general rule of thumb, I assume they can speak the casters native language, and other dependant on type and location.

Watcher are a different case again. Mostly only being able to communicate with the caster and "speak" only what they have been told/taught to.