1) Convert lat/long to Cartesian coordinates. There a zillion different projections to convert this -- you can see a list here.
For starters, you can use the simplest projection. equidistant cylindrical projection, which assumes latitude and longitude map linearly to X and Y. you can fudge with the offsets
function getXY( latitude, longitude ) {
return {x : 180 - longitude, y : latitude + 90};
}
2) Convert from "overhead" to isometric. See this question for how to rotate. isometric means that the y-coordinate is foreshortened so that the diagonal line looks as long as the side of the square, so we have to divide the y-coordinate by the multiplier.

function convertIso( x, y ) {
// m = 1 / sqrt(2);
var m = Math.sin(Math.PI/4);
// rotated would be (x*m - y*m, x*m+y*m),
// but we divide y by m to get the foreshortening
return { x : x * m - y * m, y : x + y };
}