Language selector (i18n) points to missing URLs

[Peque]

Describe the bug

If you are navigating https://example.com/en/foo/bar and have another language es configured, the language selector would link to https://example.com/es/foo/bar.

This is fine as long as /es/foo/bar exists. But this does not need to be always the case. Even with complete translations, there are some pages that are localized and only make sense in certain languages.

For those cases, the language selector should link to the first parent directory that exists (i.e.: /es/foo/ or /es/).

Reproduction

Create a project with internationalization and have one page available in one language (/en/), but not available in another language (/es/). Visit the English version and, from the drop-down language selector, the /es/ page would be linked for the Spanish version of the site, even if that page does not exist.

Expected behavior

The language selector should not link to pages that do not exist. For those cases, it could use the first existing parent or just link to the top-level language URL (i.e: /es/).

System Info

System:
    OS: Linux 6.11 Fedora Linux 39 (Container Image)
    CPU: (8) x64 Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
    Memory: 8.19 GB / 15.54 GB
    Container: Yes
    Shell: 5.2.15 - /bin/bash
  Binaries:
    Node: 20.10.0 - ~/.nvm/versions/node/v20.10.0/bin/node
    npm: 10.2.3 - ~/.nvm/versions/node/v20.10.0/bin/npm
  npmPackages:
    vitepress: ^1.5.0 => 1.5.0

### Additional context

_No response_

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- [X] Read the [docs](https://vitepress.dev).
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