Spotted while working on #58532, accounting cell formats such as _-[$£-809]* #,##0_-;\-[$£-809]* #,##0_-;_-[$£-809]* "-"??_-;_-@_- (Which should be \s£\s+12,345\s or -$\s+12,345) aren't being correctly formatted by CellFormat and DataFormatter. While we won't do anything fancy with looking up the locale code in the currency specifier, we should at least be able to spot that they're localising a $ into a £ and render as such
Basic support added in r1710484. We don't yet do anything with the locale information (though DateFormatConverter does know about them if we ever did want to), we at least now turn something like "[$£-809]" into "£" so it looks much as it does in Excel.