As I will be teaching, I will have to teach to non-native speakers as well. I usually talk slowly, but I have always related that to the fact that I am bilingual. The two languages I have grown up with keep on influencing one other. The West-Norwegian dialect has a slow pace and does not have the 'singing' which stereotypes the Scandinavian languages in general. Additionally, the tone is weak, accentless and as a consequence quite neutral. That is maybe why people find my voice monotonous. It is, however, the Flemish dialect that learned me to articulate and stress some pronunciations. This is also why I prefer to talk British English, because the accent allows me to underline what I want to say. So in communication with non-native speakers, I always use these qualities, trying to have the message understood. To chop long sentences in smaller ones may also help.
Why can't you speak ahead and do you really have to focus on the non-native speaker? He will learn the language by himself eventually? Yes and no. It is true that people will learn a language at last, but that takes time. Especially teenage immigrants are quite vulnerable for changes. The new situation (i.e. new country, new environment, different people...) is overwhelming. Plus, our education system requires the acquisition of competences in a relativly short time span, so as a teacher you will not have enough time margin to let the non-native speaker both comprehend the course contents *and* learning the language that is needed for these contents. One more important reason is the psychological backing, which gives a certain warmth, feeling of inclusion.
Now, unfortunately, there are still to many Flemings with different roots that keep on labelling themselves as the 'Moroccan' or the 'Pakistani'. The feeling of inclusion, which is partially due to good communication techniques, will automatically prompt respect for the new country and its values. I believe this is what we as a society should head to: changing immigrants into fellow citizens by the outcome of decent and well-thought communication.