I notice that the french documentation is in too many places not to understand. If I want understand, I have to look at the german or the english texts. So I have to ask for problems wich should not exist with a good translation.

Hi, please understand that only the english and german texts are from us. All other languages are translated on transifex.com from helpers. This translators must not be perfect. But everybody can help here. Also you. 

...and how can i help?

Hi
All the jDownloads Translations are done usig Transifex  Please see
http://www.jdownloads.net/documentations/item/get-transifex-account
and
http://www.jdownloads.net/documentations/item/translating-with-transifex-basics

Colin