Describe the bug

In a multilingual Hugo site with separate contentDir per language and defaultContentLanguageInSubdir: true, section pages (e.g. posts/index.html) are no longer generated for languages that don't have a _index.md in the section directory. This worked without _index.md in previous versions and appears to have regressed at some point in recent releases.

Hugo version

v0.159.1

Minimal reproduction

Config (hugo.yml):

defaultContentLanguage: en
defaultContentLanguageInSubdir: true
languages:
  en:
    contentDir: content/en
    weight: 1
  de:
    contentDir: content/de
    weight: 2

Content structure:

content/
├── de/
│   └── posts/
│       └── some-post.md        # no _index.md
└── en/
    └── posts/
        └── some-post.md        # no _index.md

Expected behavior

public/de/posts/index.html and public/en/posts/index.html are generated, as documented. The [content organization docs](https://gohugo.io/content-management/sections/) state that _index.md is optional for sections.

Actual behavior

Section index.html is not generated for one or both languages. Adding _index.md to each section directory fixes the issue.

Workaround

Add an _index.md (can be empty or with minimal frontmatter) to each section directory for every language:

content/de/posts/_index.md
content/en/posts/_index.md

Additional context

If this is intentional behavior, the documentation for [multilingual sites](https://gohugo.io/content-management/multilingual/) and [content sections](https://gohugo.io/content-management/sections/) should be updated to reflect that _index.md is required for section pages in multilingual setups with separate contentDir. Currently the docs describe it as optional, which is misleading.