Code:
function generateHreflangTags() {
var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
var lastRow = sheet.getLastRow();
for (var i = 2; i <= lastRow; i++) {
var hreflang = sheet.getRange(i, 5).getValue(); // hreflang is in column 5 (E)
var source = sheet.getRange(i, 3).getValue(); // alternate URL is in column 3 (C)
// Construct hreflang tag
var tag = '<link rel="alternate" href="' + source + '" hreflang="' + hreflang + '" />';
// Write hreflang tag in column 10 (J)
sheet.getRange(i, 10).setValue(tag);
}
}