I recently started running my first ArM saga and I've hit a snag. I introduced the concepts of elementals and ghosts from RoP:M, but now the characters want to communicate with them and I can't find a system for how they would do it. For elementals, it seems like In+Form of the elemental would be the way to go. For ghosts, I'm guessing InMe (assuming the ghost can't speak the magus's language). If these assumptions are correct, what base level would the effect(s) be?
Also, would the spells be Personal (giving the magus the ability to talk to the entity) or Touch/Voice (directly affecting the entity somehow) with penetration required?
There are several ways you could do it. Variants of Thoughts Withing Babble (pg 149 of the main book), with range personal and target hearing and an appropriate base depending on the substance allow you to understand anything of the relevant class, assuming you can penetrate its magic resistance, although that won't give them the ability to understand you. Alternatively you can use the higher base of 15 (rather than the base 5 Thoughts Within Babble uses) and get two way communication.
Alternatively, you can go for a range touch (or voice, etc.), target individual (or group etc.) to allow you to speak with a specific individual targetted by the spell. Again, this will need to penetrate magic resistance. I don't think there's a way of not needing to penetrate short of taking the heroic virtue Gift of Tongues in the Mercere section of Houses of Hermes: True Lineages - you can't change yourself to be able to speak the language without actually going out and learning the relevant language.
Base levels seem to vary by what you're talking to. For example, speaking with a natural rock (which is probably what you want for a Terram elemental) is base 20, as is speaking with a fire. Speaking with a natural body of water, the air, a human (probably what you want for ghosts) or a plant is base 15, speaking with an animal base 10. Your best bet is to go through the Intellego guidelines in the main book and pick the one that's appropriate to what you're trying to talk to. The forms other than mentem don't seem to have lower level guidelines that allow one way communication.
Because animals have simple thoughts. I would say that communicating with another human at the level at which you could communicate with an animal ("smell bad men, bad men down hill, hunt hunt!" as opposed to "later Neoplatonists embraced magical practices or theurgy as part of the soul's development in the process of the soul's return to the Source.") would be InMe level 10.
Also note that understanding another human is only level 5.