Flexibility is good, but since 'grandmother' is accepted as an answer, wouldn't that be understandable to every English speaker? Plus there are many terms from, or adapted from, other languages or unique family pet names. It could get a bit extreme if an attempt were made to cover every possibility.
But since we're adding... 'Grandma', 'Granma' and 'Nanna' (alternative spelling) are also heard in the UK.
Ah!, I see the 'translation' above is given as 'Your nan'. That should have been 'Your grandmother' but accepted the major variants, I suppose. It gave 'Grandmother' in the hints.
'Nan' is actually what we used within my family, but I would have said 'Grandmother' to anyone outside the family, or in writing.
That's pretty much how we use kinship terms in my family.
I address my Father as "Dad" and only refer to him as dad when speaking with my siblings, otherwise I refer to him as "My father", and other people's father as "Your/their Father". (You would catch me saying "Your dad", except in very special circumstances)
The same holds true for "Mother/mum", "Grandfather/Granddad", and "Grandmother/Grandma".
The spanner in the works "Nana", this is what my father called his Welsh grandmother, so he insisted that us kids refer to her as Nana as well.
Every family has their quirks I guess.