feat: use en-US as fallback recommend app if using unmaintained language

[takatost]

Description

use en-US as fallback recommend app if using unmaintained language

Type of Change

• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Manual test

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods
• optional I have made corresponding changes to the documentation
• optional I have added tests that prove my fix is effective or that my feature works
• optional New and existing unit tests pass locally with my changes