That being said, having read several biographies of King Ludwig II of Bavaria, I have largely become resigned to authors referring to his mother as the "Queen Mother" -- despite the fact that this title (to the best of my knowledge) never got formally used in Germany. After all, that's what Marie technically was, after obtaining dowager status in 1864
It's just that I wish the said biographers would place the words in lower-case letters, since "queen mother" is largely a descriptive term. I believe she was the only queen of Bavaria to meet the qualifications for one (i.e. the widow of a king who was also the mother of another).