Not exactly the best idea...

...but you can use this to Google Translate your .vtt subtitle files into other languages!

Requires Node.js 7.10.1 or later.

I really want to emphasize that this isn't a great way to produce a translation for your video/audio content. The best way is to hire a professional translator. If you have a budget for producing your content, and you need a translation, please allocate some of that budget for paying a translator.

If you're working on a volunteer-based project with no budget, or you just need some test data, this can work well enough for you.

Keep in mind that translations are done in fixed sized batches of a certain number of lines, so there probably will be occasional translation errors specifically due to context being cut off from the beginning or end of a batch of text.

npm install --global vtt-translate

$ vtt-translate 
Input VTT filename: ./vtt/transcript_en.vtt
Source language code: en
Destination language code: fr
Google Translate API key (https://cloud.google.com/translate/): [Paste your API key here]
Translating...
Translation complete!
Destination VTT filename: ./vtt/transcript_fr.vtt
$

See the Node API description for explanation of the command line inputs.

Note that API limits apply... if you fill your daily quota, a restore file will be saved so you can continue the next day without losing your progress:

$ vtt-translate 
Input VTT filename: ./vtt/transcript_en.vtt
Source language code: en
Destination language code: fr
Google Translate API key (https://cloud.google.com/translate/): [Paste your API key here]
Translating...
{ code: 403,
  message: 'User Rate Limit Exceeded',
  errors: 
   [ { message: 'User Rate Limit Exceeded',
       domain: 'usageLimits',
       reason: 'userRateLimitExceeded' } ],
  translatedCues: 
   [ ... ] }

Saving temp file so we can resume later where we left off...
Progress: 2688 of 2767 cues (2688 newly translated)
$

Don't remove this temp file! Otherwise you'll have to start over from scratch later.

Once the download completes, the temp file will be deleted automatically.

npm install --save vtt-translate

This API example is functionally identical to the earlier command line example.

const { getTranslatedVttForPathname } = require('vtt-translate');
const fs = require('fs');

const pathname = './vtt/transcript_en.vtt';
const apiKey = '[Paste your API key here]';
const source = 'en';
const target = 'fr';

getTranslatedVttForPathname(pathname, apiKey, { source, target })
  .then(vttContent => {
    fs.writeFile('./vtt/transcript_fr.vtt', vttContent, err => {
      if (err) {
        console.error(err);
      }
    });
  })
  .catch(err => {
    console.error(err);
  });

• pathname: The filesystem pathname for the source vtt file.
• apiKey: A valid Google Cloud Translation API key
• params.source: A language code supported by Google's Neural Machine Translation Model, corresponding to the language of the source text (e.g. 'en')
• params.target A language code corresponding to the translation target language
• params.restoredCuesPathname: A pathname pointing to a JSON file saved with a backup of previously translated cues (see error handling)

A Promise which resolves with the full text of the translated vtt file (as a string)

If you .catch an error and the error has a translatedCues property, you can save that object to a json file and restore it later with params.restoredCuesPathname. This allows downloading a translation across multiple days if your API request quota is too small to download the translation in one day.