fateforge-tool/js/spells.js

var normaliseLang;
normaliseLang = function (lang) {
  lang = lang.replace(/_/, "-").toLowerCase();
  if (lang.length > 3) {
    lang = lang.substring(0, 3) + lang.substring(3).toUpperCase();
  }
  return lang;
};

$.defaultLanguage = normaliseLang(
  navigator.languages && navigator.languages.length > 0
    ? navigator.languages[0]
    : navigator.language || navigator.userLanguage
);
var lang = normaliseLang( $.defaultLanguage); 

fetch('data/spells/spells-grimoire-fr.json')
    .then((response) => response.json())
    .then((json) => JSON.parse(json));


var string jsonsource = ''.concat("../data/spells/spells-grimoire-",navigator.languages[0].slice(0,2),".json");

import data from jsonsource assert { type: 'json' };
console.log(data);

var yourHtml = JSON;

document.getElementById("spell-list").innerHTML = yourHtml;