const allLanguages = [ 'ES', 'EN', 'DE' ]
const usedLanguages = [ { id: 1, lang: 'EN' } ]

const availableLanguages = [ 'ES', 'DE' ]

const all = new Set(allLanguages);
const used = new Set(usedLanguages.map(({lang}) => lang));

const availableLanguages = setDifference(all, used);

const setDifference = (a, b) => new Set([...a].filter(x => !b.has(x)));

const not = fn => x => !fn(x);
const isIn = set => x => set.has(x);

const setDifference = (a, b) => new Set([...a].filter(not(isIn(b))));

const setDifference = (a, b) => {
  const result = new Set(a);
  b.forEach(x => result.delete(x));
  return result;
}

const availableLanguages = all.difference(used)

function* difference(array, excludes) {
  for (let x of array)
    if (!excludes.includes(x)) yield x;
}

console.log([...difference(allLanguages, usedLanguages)]);

const dict = Object.assign({},
    ...usedLanguages.map(({lang}) => ({[lang]: true})));

const availableLanguages = allLanguages.filter(lang => lang in dict);

const dict = usedLanguages.reduce((obj, {lang}) => {
  obj[lang] = true;
  return obj;
}, {});

const dict = usedLanguages.reduce((obj, {lang}) => (obj[lang] = true, obj), {});

const dict = {};
for (x of usedLanguages) {
  dict[x.lang] = true;
}

availableLanguages = allLanguages.filter((lang1) => !usedLanguages.some((lang2) => lang2.lang === lang1))