Please note that this has already been addressed by the Germany Project (though you will have to be able to read German to see the policy). The German-language policy page is here; it is also referenced (though not properly) on the English-language policy page here.
In essence, the policy states that the English-language structure will end at the municipality level, but the German-language structure will have an additional level below, of the form Village, State - and in the case of possible ambiguity, Village (Municipality), State. The village-level categories (in German only) are to be placed under the municipality-level categories in both the English and the German categories.
Thus, for example, Morschenisch, Nordrhein-Westfalen, and Girbelrath, Nordrhein-Westfalen (both village-level categories) are to be placed under Gemeinde Merzenich, Nordrhein-Westfalen (municipality-level structure).
In your particular case, there should be a German-language category Schönenbach, Baden-Württemberg placed under both the German-language and English-language categories Furtwangen, Baden-Württemberg.