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 "language-phobic" 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.

I don't think that you can paper over the hindrance by formulating the job offer in a particular way.

I live in a country neighboring a big country that tends to force everything in the native language. There are plenty of job opportunities there, but many of them would require me to teach in the local language. If I were to apply to one of those, this has the following disadvantages:

• I need to learn the local language;
• I need to translate all my teaching materials into the local language;
• the students will not be challenged to learn English, so instead of facing a challenge head-on, they get to take the easy way out;
• the students will want to write their master's thesis in the local language too, so I would need to translate all their work into English if I would want to derive joint publications from it;
• the students will have learned all technical terms in the local language, which puts them as a disadvantage when they enter the international job market.

There are plenty of countries on this planet that do not force me to deal with this situation. The situation forces me to do all kinds of unnecessary extra work, and the only benefit is that the students will be allowed to take the easy way out, their work will have a smaller reach, and the university graduates will be of lower value to the international market. Why on Earth would I apply? No matter how much linguistic support you throw at it, I would always prefer to teach in Canada, or Ireland, or Australia, or New Zealand, or Scandinavia, or...

I think if I needed to learn a language well enough to teach in it in two years then that would basically mean I spent those two years learning that language as a full-time job and getting very little research done. I could imagine making that substantial sacrifice for the right job in the right location, but it's a very big hurdle. This is on top of already having a big adjustment moving to a country you're unfamiliar with. Unless the position is clearly better in some way (pay, teaching load, location) it's going to be a difficult sell.