_Configuration:_
- Web browser and its version: 
   - **Edge**: 91.0.864.64 
   - **Chrome**: 91.0.4472.124
- PDF.js version:
   - Latest demo version: https://mozilla.github.io/pdf.js/web/viewer.html

_Steps to reproduce the problem:_
1. Open the demo viewer in Chrome
2. Open the demo viewer in Edge

_What is the expected behavior?_ 

The viewer should be localized, i.e. translated to Swedish.

_What went wrong? (add screenshot)_

The viewer is translated to Swedish in Chrome, but not in Edge.

_Request headers:_
  - Chrome: `Accept-Language: sv-SE,sv;q=0.9,en-US;q=0.8,en;q=0.7`
  - Edge: `Accept-Language: sv,en;q=0.9,en-GB;q=0.8,en-US;q=0.7`

The problem seems to be that the `l10n` folder does not contain an `sv` folder, but only an `sv-SE`. 
if (!args) {
    return text;
  }
  return text.replace(/\{\{\s*(\w+)\s*\}\}/g, (all, name) => {
    return name in args ? args[name] : "{{" + name + "}}";
  });
}
/**
 * No-op implementation of the localization service.
 * @implements {IL10n}
 */
const NullL10n = {
  async getLanguage() {
    return "en-us";
  },
  async getDirection() {
    return "ltr";
  },
  async get(key, args = null, fallback = getL10nFallback(key, args)) {
    return formatL10nValue(fallback, args);
  },
  async translate(element) {},
};
export { getL10nFallback, NullL10n };