Was it really so widely used in Germany?


There are two dimensions to look at: the regional one, and the social one.

Regional: Was the Allemande worn everywhere in Germany, and was it worn anywhere outside Germany?

The paintings I've seen portray ladies from Prussia, Westfalia, Thuringia, Saxony, Palatine, Bavaria and even from Gdansk, which belonged to Germany at the time. So the answer to part one of the question is yes.
Meta Klopstok,
the poet's wife

As for part two, the answer is a conditional yes. The variations that fulfill all the criteria for French-style court dress were definitely worn in Italy, Austria, Denmark, Sweden and Russia. One picture I've received from a benefactor was from Ireland. The variations that do not fulfill all the criteria of French court dress have, until now, only been found in Scandinavia and, in one case, in North America. If you find an example anywhere outside Germany, please take note of as many details as possible (portraitee, painter, year of creation, present location) and let me know!

Social: Who wore the Allemande?

The court dress in the strict sense was of course worn by royals, but also by ladies who attended at royal courts, such as ladies in waiting. They were invariably members of the nobility. We also see it on noble ladies outside any royal court and in one single known case on a non-noble woman, the wife of a famous poet who, thanks to the fame of her husband, may have been invited to court now and then. It would only be natural for her to wear her best (i.e. court) dress to sit for a painting.

So as far as I know at this point, the Allemande was restricted to royalty, nobility, and those members of the lower ranks who interacted with the former.

Next chapter: Why does it appear so often on German ladies, but not on French, English
etc ones?