My understanding is that in October 1990, when East and West Germany came back together one condition was that they join the Euro currency.
At that time the Deutsche mark was very strong and was being used in countries like Yugoslavia where the currency was unstable,
The gold holdings of Germany/France etc were kept outside the Euro and still are not being shared.
Germany, France and Italy have large gold holdings.
Germany holds 3,350 tonnes of gold, which is valued at about €440 billion US, much of it still stored in the USA.
So if Germany did now leave leave Euroland the Deutsche mark would again be the strongest currency and possibly be used in other currencies.
Quite a serious problem, I thought it might pop up one day.
If you know more please let me know. (Or correct me)