How to ensure adequate dissemination and wording of an academic job offer when the teaching language will not be English?
You cannot change the location of your university and the fact that the local language is not English. Moreover, the question focuses on tenure-track positions, so I will interpret that as positions where the goal is that the holder stays for a longer period of time (lets arbitrarily say 10 years), but correct me if I am wrong. Even if teaching in English was not a requirement, the holder would still need to learn the local language. You can survive in many places with only knowing English, but to make the best use of everything that is possible you often need to know the local language. So, in practice I find that the requirement to teach in the local language, with the accompanying support, actually helps foreign position holders adept to their new environment. So there is no need to be defensive about that.
Regarding your first question: I would say that there is continuum of candidates where on the one extreme are people who enjoy experiencing new places, and for whom learning the language is less of a chore and more part of the experience, and on the other extreme are other people for whom learning a new language is going to be too much. Your university will have trouble attracting people who are more on the &quot;language-phobic&quot; side of the spectrum, but that is not a bad thing as they would not be a good match for your university anyhow.
Regarding your second question: keep it short, to the point, and factual. Those who are not daunted by learning a new language, just want to know whether the requirements are reasonable for them. Is there a transition period, and if yes how long? What kind of support is there? In the advertisement, that needs to be very short, but you could include a link to a place that provides more information. Again, don't be defensive about it.