feat: add React multilingual intranet switcher sample

[vystartasv]

Description

Adds a read-only React multilingual intranet switcher sample for SPFx 1.23.2.

What changed

• Uses bounded local JSON configuration for locale-specific navigation labels and descriptions.
• Selects exact browser locale, language fallback, then configured default.
• Requires explicit user selection through an accessible Fluent UI language selector.
• Accepts only same-origin HTTP(S) links and rejects unsafe/external URLs.
• Performs no translation, API calls, SharePoint reads, or tenant mutations.

Validation

• npm ci
• npm test — 6 Mocha tests passed; SPFx lint, TypeScript, Sass, and webpack validation passed.
• gulp bundle --ship — passed.
• gulp package-solution --ship — production .sppkg created successfully.
• Diff, metadata, config, and secret scans passed.

A non-fatal source-map warning appears in the DEBUG pipeline from generated SCSS mapping; SHIP bundle and package complete without it.

Tenant validation requested

Please test with two or more configured locales on a SharePoint Online site, including managed paths, unsupported browser locales, malformed configuration, unsafe links, keyboard navigation, responsive layout, and users without special API permissions. Real tenant screenshots are welcome; none are fabricated locally.

[vystartasv]

Could someone with a SharePoint Online tenant please validate PR #6484 with two or more configured locales and add real screenshots if possible?

Please check:

• Exact locale selection, language fallback, and configured-default fallback.
• Managed-path SharePoint URLs.
• Unsafe, malformed, protocol-relative, and external URL rejection.
• Malformed or oversized configuration behavior.
• Keyboard navigation, visible focus, responsive layout, and screen-reader labels.
• Behavior for users without additional API permissions.

This sample is deliberately read-only and configuration-driven. It does not translate content or call a translation service. No tenant screenshots are fabricated locally.

[github-actions]

Validation failed!

File: samples/react-multilingual-intranet-switcher/assets/sample.json

• Missing 'thumbnails' property!

[github-actions]

Sample PR validation for #6484

⚠️ Validation status: warnings

We automatically validate all pull requests against our contribution guidance to ensure that all samples provide a consistent experience to our community.

In order to merge this PR in a timely manner, the following criteria must be met:

Validation	Status
Pull request affects only one folder	✅ Succeeded
Sample folder name follows naming convention	✅ Succeeded
README.md contains visitor stat image	⚠️ Warning
Sample requires a .nvmrc file	✅ Succeeded
Sample requires a README.md	✅ Succeeded
Sample requires a screenshot .png in assets folder	⚠️ Warning
No .sppkg file	✅ Succeeded
No node_modules folder	✅ Succeeded
No lib folder	✅ Succeeded
No upgrade reports	✅ Succeeded

@vystartasv please address the above issues and push new changes to this branch.
For more information, see the contribution guidance.

[github-actions]

Sample PR validation for #6484

⚠️ Validation status: warnings

We automatically validate all pull requests against our contribution guidance to ensure that all samples provide a consistent experience to our community.

In order to merge this PR in a timely manner, the following criteria must be met:

Validation	Status
Pull request affects only one folder	✅ Succeeded
Sample folder name follows naming convention	✅ Succeeded
README.md contains visitor stat image	⚠️ Warning
Sample requires a .nvmrc file	✅ Succeeded
Sample requires a README.md	✅ Succeeded
Sample requires a screenshot .png in assets folder	⚠️ Warning
No .sppkg file	✅ Succeeded
No node_modules folder	✅ Succeeded
No lib folder	✅ Succeeded
No upgrade reports	✅ Succeeded

@vystartasv please address the above issues and push new changes to this branch.
For more information, see the contribution guidance.

[github-actions]

Sample PR validation for #6484

⚠️ Validation status: warnings

We automatically validate all pull requests against our contribution guidance to ensure that all samples provide a consistent experience to our community.

In order to merge this PR in a timely manner, the following criteria must be met:

Validation	Status
Pull request affects only one folder	✅ Succeeded
Sample folder name follows naming convention	✅ Succeeded
README.md contains visitor stat image	✅ Succeeded
Sample requires a .nvmrc file	✅ Succeeded
Sample requires a README.md	✅ Succeeded
Sample requires a screenshot .png in assets folder	⚠️ Warning
No .sppkg file	✅ Succeeded
No node_modules folder	✅ Succeeded
No lib folder	✅ Succeeded
No upgrade reports	✅ Succeeded

@vystartasv please address the above issues and push new changes to this branch.
For more information, see the contribution guidance.