I was going by the example given rather than the initial question posted (which, when looking at the example, asks a very different question), sorry if I have offended in any way. I figured you answered the first part well enough, but didn't answer the actual example, which is what I wanted to clarify.

How do we know that, in the case of watchers, it isn't by rote memorization? While a watcher can communicate telepathically with its summoner, the "Astral Messenger" service seems to be limited to a phrase/picture and a response. All the watcher would have to do is mimic the sounds coming out of the summoner's mouth, and then "record" the response once he has delivered the message. Telepathic communication with the summoner is one thing, and communication through language is another.

Again, I can see kind GMs and Gaming Groups allowing all spirits to know the native language of their summoner, and being able to speak said language fluently (the equivalent of 2 points of the native language skill). This brings up issues, though, of translation... if a spirit speaks to, say, a German Wiccan who only speaks German, would it be able to communicate at anything beyond an emotional and psychic level? If it CAN, why is it that there are no spirit interpreters, as they would be able to translate any human language, apparently?

Back to the question of the Ally Spirit, it isn't specifically stated that the Ally knows any language unless it is given to them by the summoner. The only communication that exists is the telepathic bond between the summoner and itself. I'm sure the Ally can understand spoken language (especially if it has been around for a while), but to respond intelligently in a spoken language on the astral plane is another thing entirely.

A non-canon possibility is that all language on the astral plane is being-to-being, mind-to-mind, a telepathic connection between two entities like Dragon thoughtspeak. This again brings up issues of translation of languages (why no astral interpreters?), but it conveniently sidesteps the issues that underlies the native language problem.

There's no basis in the book that they have meaningful speech with people other than their summoner, either. It is clear that spirits and summoners of those spirits can communicate in some way (most likely thought-voice or telepathy). It is never made clear whether or not a watcher can have meaningful communication to other spirits or to other people (especially those who speak a different language from the summoner). Thoughtvoice is one thing, speaking in Spanish or Sperethiel is another.