Converter coordinates, vertical orthometric & horizontal, js + python

I need a developper to create a little tool that :
- convertcoordinates from WGS84 + Elipsoid to UTM 40S + Orthometric
- from coordinates UTM40S + Orthometric to WGS84 + ellipsoid

the tool should have 2 buttons (no design requiert) :
one to perform it in JS and the other to do the same in python (or any backend langage)

Note : the vertical converter is very tricky, and requiert either grid file or tiff file to be able to make the convert.

Explain to me your approach clearly.