Non-ASCII characters in the slug may cause navigation and routing to break

[joulev]

Link to the code that reproduces this issue

https://github.com/joulev/debug/tree/nextjs-with-non-ascii-chars

To Reproduce

Simply start the application in either dev mode or prod mode or on Vercel. Vercel prod mode deployment of the reproduction repo above is already available on https://debug-git-nextjs-with-non-ascii-chars-joulev-proj.vercel.app.

Current vs. Expected behavior

Current behaviour

	Server redirect	Client router.push	next/link
Local next dev	❌ Error	❌ 404	❌ 404
Local next start	❌ Error	❌ 404	❌ 404
Vercel	❌ Error	✅	✅

Expected behaviour: All 9 cases above should work.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.2.0: Fri Dec  6 18:51:28 PST 2024; root:xnu-11215.61.5~2/RELEASE_ARM64_T8112
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 22.12.0
  npm: 10.9.0
  Yarn: N/A
  pnpm: 9.15.0
Relevant Packages:
  next: 15.1.0 // Latest available version is detected (15.1.0).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Navigation

Which stage(s) are affected? (Select all that apply)

next dev (local), next start (local), Vercel (Deployed)

Additional context

No response