Mediating inter-species languages, which allow to use the mechanical / vocal properties of the two species optimally for communication; then teach them those languages and make the communication possible. For example, dolphins can deal with human languages quite well (even with second order logic reasoning), but it takes too much mechanical effort, since their own language is quite faster and higher pitch. So some medium is needed. But I feel that we would most likely opt for machinery doing the hard translating mediation between us, first, and only then strive for the real deal. (obviously no machinery, and the two species using the same language directly, is preferred)