The main characteristic of such in house training programs is that they are relatively short. Thus you won't have time to teach complex skills like writing or public speaking and should focus more on policies and procedures.
The first thing I would cover, in cooperation with human resources, is sensitivity and multicultural awareness. Managers should be trained in using gender neutral terms, and avoiding references that might be seen as cultural prejudice (i.e. "winter holiday/new year" rather than Christmas). Next, managers should be trained in the communication styles appropriate to different cultures, so that they can communicate effectively to customers and suppliers in a global economy. Finally, managers should be trained in company policy and procedures concerning e-mail and social networking.
If additional time permits, I would have each manager draw up a communication plan for his or herself, covering what needs to be communicated when (reports up the hierarchy, and perhaps weekly or monthly updates to subordinates) and in what formats.