If the titles are used correctly there never can be any confusion.
In the UK EII is HM the Queen.
her mother would have been HM Queen Elizabeth and the widow of Albert I of Belgium HM Queen Elisabeth of Belgium.
In Belgium the dowager queen was HM Queen Elisabeth, EII HM the Queen of the United Kingdom and Northern Ireland and her mother HM Queen Elizabeth of the United Kingdom and Northern Ireland.
Indeed, I'm beginning to suspect that the British case was the exception, not the rule. It's highly doubtful that the mother of QEII would have styled herself as Queen Mother, if she had had a different name from that of her sovereign daughter.
That being said, if this title were used in Belgium, and had not King Leopold III abdicated the throne, then Europe would have had a period in the 20th century with two queen mothers named Elizabeth. Of course, the spellings are different; but it still could have created confusion.
There were, in fact, three women in Europe named Elisabeth/Elizabeth who were styled with the title of Queen, 1952-1965. So there probably was indeed a lot of confusion ("which one?").