Translating words from one language to another
Personally, I would have the translate call return a Promise rather than a callback and make use of async/await (presuming the version of Node you are using Node 6+) e.g.

var express = require('express');
var request = require('request');

var router = express.Router();

router.post("/", async (req, res, next) =&gt; {
  try {
    const translated = await translate(req.query.source, 'en', req.query.content);
    const result = await translate('en', req.query.target, translated); 
    // presume you intend on returning something from your API?
    return res.json({ result });
  } catch (e) {
    return next(e);
  }
})

const translate = (source, target, content) =&gt; 
  new Promise((resolve, reject) =&gt; 
    request(`https://api-platform.systran.net/translation/text/translate?input=${content}&amp;source=${source}&amp;target=${target}&amp;key=xxxxxxxx-783f-4f90-aea4-7fb357016647`, (err, data, body) =&gt; {
      if (err) return reject(err);

      try {
        return resolve(JSON.parse(body));
      } catch (e) {
        return reject(e);
      }
    })
  );

module.exports = router;