[DOCS] Show translation completeness and language status in docs

[bnbong]

Summary

Improve documentation trustworthiness by showing language completeness/status instead of implying that all configured languages are equally complete.

Scope

• Add a visible language status/completeness note in docs
• Clarify that English is the source of truth unless another policy is explicitly documented
• Update translation contributor docs accordingly
• Prefer transparency over broad language claims

Out of Scope

• Completing all translations
• Removing language builds unless clearly necessary

Acceptance Criteria

• Users can easily understand translation completeness expectations
• Docs no longer over-promise parity across all configured languages
• Translation contributor docs explain the status model clearly
• The change works cleanly with the current MkDocs i18n setup

Suggested Areas

• docs/en/index.md
• docs/en/contributing/translation-guide.md
• mkdocs.yml