How can mature Wikipedias like English and German support the development of local language Wikipedias?
The most obvious way is by technical and information support. That means help about using sophisticated techniques, complicated templates, etc. and a coordinating project for local language editions, whose active users speak the larger language (German and English, as the question puts it). There is some positive experience in Russian language project ru:Википедия:Проект:Малые разделы Википедии на языках России (linked from the main page at ru.wiki). Amikeco 07:45, 1 November 2009 (UTC)