It is impossible to use [NatNum12] with a fractional number format. E.g., put 0.4 to a cell in Calc, and format the cell using format string "0/0", and it would show "2/5". One could expect that "[NatNum12]0/0" would allow to get (in en-US) "two fifths" or the like, but it shows "two/five" instead.
Repro Arch Linux 64-bit, X11 Version: 7.6.0.0.alpha0+ (X86_64) / LibreOffice Community Build ID: fba19e93cca9a7cdfa1e4ac92291cd0b7f06d715 CPU threads: 8; OS: Linux 6.2; UI render: default; VCL: kf5 (cairo+xcb) Locale: fi-FI (fi_FI.UTF-8); UI: en-US Calc: threaded Built on 24 March 2023