const results = [

  {

    rating: 5,

    title: { da: 'Web udvikling', en: 'Web Development' },

    other: [1, 2, {}],

    nothing: null,

    fn: function() {}

  },

  {

    rating: 5,

    title: { da: 'Node.js', en: 'Node.js' },

    foo: {

      bar: {

        baz: { fr: 'Bonjour', en: 'Hello' }

      }

    }

  }

];

​

const reduceByLang = (data, lang) => {

  // Look for a `lang` key in obj or

  // if not found but still an object, recurse

  const reduceByLangObj = (obj) => {

    Object.keys(obj).forEach((key) => {

      if (obj[key] === null) {

        return;

      }

      if (obj[key][lang]) {

        obj[key] = obj[key][lang]; // replace with desired lang

      } else if (typeof obj[key] === 'object') {

        reduceByLangObj(obj[key]); // recurse

      }

    });

    return obj;

  };

  if (Array.isArray(data)) {

    return data.map(reduceByLangObj);

  } else {

    return reduceByLangObj(data);

  }

};

​

console.log(reduceByLang(results, 'da'));

console.log(reduceByLang(results, 'en')[1].foo.bar.baz);