Add new style for German journal "Deutsche sprache"

The guidlines are quite short and let some issues unclear. Therefore, I checked the style also by looking at a current article closely. (It is very probable that this was asked in https://forums.zotero.org/discussion/39057/citation-style-request-deutsche-sprache/ )
this would need an ```et-al-use-first```. Also, no et al's for any other editors?