_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`. 
rendering_error: "An error occurred while rendering the page.",
  page_scale_width: "Page Width",
  page_scale_fit: "Page Fit",
  page_scale_auto: "Automatic Zoom",
  page_scale_actual: "Actual Size",
  page_scale_percent: "{{scale}}%",
  loading: "Loading…",
  loading_error: "An error occurred while loading the PDF.",
  invalid_file_error: "Invalid or corrupted PDF file.",
  missing_file_error: "Missing PDF file.",
  unexpected_response_error: "Unexpected server response.",
  printing_not_supported:
    "Warning: Printing is not fully supported by this browser.",
  printing_not_ready: "Warning: The PDF is not fully loaded for printing.",
  web_fonts_disabled:
    "Web fonts are disabled: unable to use embedded PDF fonts.",
};
function getL10nFallback(key, args) {
  switch (key) {
    case "find_match_count":
      key = `find_match_count[${args.total === 1 ? "one" : "other"}]`;
      break;
    case "find_match_count_limit":
      key = `find_match_count_limit[${args.limit === 1 ? "one" : "other"}]`;
      break;
  }
  return DEFAULT_L10N_STRINGS[key] || "";
}
// Replaces {{arguments}} with their values.
function formatL10nValue(text, args) {