Could this return a truefalse string too, that's configurable?
E.g. like this (but not actually this, it's a bit ugly and things might be undefined, obviously):
`return (o.value?o.column.booleanFormat.trueString || "true":o.column.booleanFormat.falseString || "false");`

So that I could have 'yes'/ 'no' or 'si'/'non' etc.  To keep behaviour as it is in the implementation at the moment, default values for the strings could both be " ".