IMO:
Spirit Speech allows you to communicate accurately with spirits, most of whom don't speak English or Garou. The exact manner of communication varies by spirit type, but anyone without the Gift is rarely able to understand any communication any more than a normal human would be able to understand a communication in some totally unknown language, which is to say, unless active subterfuge is involved, you can probably guess at which things are greetings and which things are profanity and the general emotional state of the people involved, and if you're lucky you might catch untranslatable things like names, but any detail past that is extremely unlikely.
Spirit Speech does not allow two Garou to speak directly to each other. I have two phones, and they each have a charger, and those chargers both plug into this wall, but I cannot just shove the two plugs into each other and charge one phone directly off of the other. A third-party spirit can be brought in to translate between two people, but there is no particular guarantee that this translation will be accurate, or that the spirit involved will not choose to be a dick about it.
Either way, Spirit Speech is not a single language. It's a magical power that lets you communicate concepts with spirits, each type of which generally has their own approach to communication. Some are auditory, some are movement-based, some spirits probably communicate entirely through calibrated biting, etc. But a new Garou seeing someone use it for the first time is probably weirded out, and using it in view of humans is about as veil-breachy as performing a rain dance on main street, in that if nothing happens it's weird, and if something happens it's VERY weird.
Mystical stuff - it's weird!