_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`. 
document_properties_page_size_name_a3: "A3",
  document_properties_page_size_name_a4: "A4",
  document_properties_page_size_name_letter: "Letter",
  document_properties_page_size_name_legal: "Legal",
  document_properties_page_size_dimension_string:
    "{{width}} × {{height}} {{unit}} ({{orientation}})",
  document_properties_page_size_dimension_name_string:
    "{{width}} × {{height}} {{unit}} ({{name}}, {{orientation}})",
  document_properties_linearized_yes: "Yes",
  document_properties_linearized_no: "No",
  print_progress_percent: "{{progress}}%",
  "toggle_sidebar.title": "Toggle Sidebar",
  "toggle_sidebar_notification2.title":
    "Toggle Sidebar (document contains outline/attachments/layers)",
  additional_layers: "Additional Layers",
  page_landmark: "Page {{page}}",
  thumb_page_title: "Page {{page}}",
  thumb_page_canvas: "Thumbnail of Page {{page}}",
  find_reached_top: "Reached top of document, continued from bottom",
  find_reached_bottom: "Reached end of document, continued from top",
  "find_match_count[one]": "{{current}} of {{total}} match",
  "find_match_count[other]": "{{current}} of {{total}} matches",
  "find_match_count_limit[one]": "More than {{limit}} match",
  "find_match_count_limit[other]": "More than {{limit}} matches",
  find_not_found: "Phrase not found",
  error_version_info: "PDF.js v{{version}} (build: {{build}})",
  error_message: "Message: {{message}}",
  error_stack: "Stack: {{stack}}",
  error_file: "File: {{file}}",
  error_line: "Line: {{line}}",