[w3c/ServiceWorker] Service worker cache localisation (#1453)

_Sorry this is not really an issue but a question / problem I was having and wasn't sure where to ask._

I'm trying to implement a service worker that will sit at the root of our website however it will need to cache assets that it is unaware of. They are coming from a collection of domain specific microservices and I don't want teams to need to release SW as part of their deployment pipeline.

Todo this when each service is built it creates it's own asset manifest similar to this:

```
{
    application: {
        name: 'app name,
        id: 'app-id
    },
     css: [
          { filename: 'name.hash.css' }
     ],
     js: [
          { filename: 'vendor.hash.js' }
     ],
     fonts: [
          { filename: 'font.woff2' }
     ]
 }
```

There is then a service that collects all our services assets manifests and generates a sw cache manifest:

```
{
    'app-id': ['/asset/name.hash.css','/asset/vendor.hash.js', '/asset/font.woff2']
    ...
}
```

This is uploaded along with our assets to a bucket that is internet facing. Then our service worker loads in this json file at boot time and passes a merged array of assets to `workbox.precaching.precacheAndRoute`.

This means that our service worker doesn't need to have an understanding of what our separate teams are releasing, or even if we add a new service and the cache can always be upto date. However my issue does in because we also support multiple languages.

This means I want to load different assets into the cache based on the users set language, I looked at a few different options:

- Use browser language setting: https://chromium-review.googlesource.com/c/chromium/src/+/1136794 - however browser language doesn't mean its the language they are browsing the site.
- You can't access cookies in service worker: https://github.com/w3c/ServiceWorker/issues/707 however you could potentially read the language in the website and use post messages to send it back to service worker: https://googlechrome.github.io/samples/service-worker/post-message/ - this seems pretty gross though and I've not tried this.

So my final thought was to register a service worker dynamically based out the url:

```
$ curl https://www.my-site.com/en-gb/

if ('serviceWorker' in navigator) {
  window.addEventListener('load', function() {
    navigator.serviceWorker.register(`/{en-sw.js`);
  });
}
```

From here then the service worker could call my cache manifest which could return something like:

```
{
    'app-id':{
        en: ['/asset/name-en.hash.css','/asset/vendor-en.hash.js', '/asset/font.woff2'],
        de: ['/asset/name-de.hash.css','/asset/vendor-de.hash.js', '/asset/font.woff2']
    }
}
```
Then it would know as the `en-sw` to use the `en` array.

However this would involve either generating multiple service workers (one per language) or service workers are actually generated programatically based on the in bound request.

At this point I'm not entirely sure if I'm over thinking this and there is a much simpler way of doing it... so just wondering if this is a problem anyone else has had and how they solved it.

Thanks,
Robin

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