getStaticPaths with fallback: false, return 404 page for non-ASCII/non-Latin url

Bug report

Describe the bug

In the following page called pages/a/[slug]/index.js

export async function getStaticPaths() {
  return {
    paths: [{ params: { slug: 'Synonyms' } }, { params: { slug: 'อักษรย่อ' } }],
    fallback: false,
  };
}

export async function getStaticProps({ params }) {
  const res = await fetch(getApplicationDetailBySlug(params.slug));
  const app = await res.json();

  return {
    props: { app },
  };
}

The value of slug is dynamic value that can be ASCII or Non-ASCII characters, So, When navigate to url of ASCII (Synonyms), Page rendered correctly but for Non-ASCII (อักษรย่อ), I got 404 page.

List of urls:

• อักษรย่อ Url: /a/อักษรย่อ encodeURIComponent: /a/%E0%B8%AD%E0%B8%B1%E0%B8%81%E0%B8%A9%E0%B8%A3%E0%B8%A2%E0%B9%88%E0%B8%AD

• Synonyms Url: /a/Synonyms encodeURIComponent: /a/Synonyms

To Reproduce

Steps to reproduce the behavior, please provide code snippets or a repository:

1. in getStaticPaths(), Use url param with non-ascii characters, I think for all non-latin language will face this issue, e.g. Thai, Chinese, Japanese.
2. fallback: false
3. browse the url in browser.
4. See 404 error page.

Expected behavior

Should return correctly of url path and param for all encoding in fallback: false mode.

Screenshots

Page of non-ascii url:

Page of ascii url:

Result from build command

Files from build command in .next folder:

System information

• OS: Windows 10 Pro (WSL Bash shell)
• Version of Next.js: 9.5.3
• Version of Node.js: v12.18.4

Additional context

OSX is got same behavior and 404 page. So, Path syntax is not root cause for this case for sure.

[simpleasnothing]

same issue

[balazsorban44]

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