From the grapevine this has more to do with the international stream exerting more influence and potentially changing the school a little bit....
The German speaking "rule" would prevent these international board members from exerting proper (or perhaps improper, from the other side's perspective) influence on matters which will come up for vote.
Some might say whoever pays the bills has the largest vote.... but what do I know.The fairest thing would be that a number of non-German speakers can stand in proportion to their % of the school.
Speaking a language is a skill, it has nothing to do with race. Plenty of jobs are advertised with requirements of speaking English, Cantonese mandarin etc... If they required the person to be a German national, that would be another matter. Is it an issue for ESF schools to require that people speak English?