Add external translation provider workflow guide

[mintlify]

Summary

Adds documentation for teams using their own translation providers (external vendors, regional translators) to work with Mintlify documentation.

• Workflow overview for export/import translation cycles
• GitHub Actions examples for automating content export and translation import
• Best practices for working with external translators (frontmatter, code blocks, glossaries)

Addresses the need outlined in DOC-263 for customers like Notion who work with different regional translators and need guidance on getting MDX files to/from their translation providers.

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Note

Low Risk
Low risk: documentation-only updates that don’t change runtime code paths. Main risk is users copying the example GitHub Actions workflows without adapting language paths and validation to their repo.

Overview
Adds an external translation provider section to guides/internationalization.mdx, outlining an export/send/receive/import workflow for working with third-party translators.

Includes copy-pastable GitHub Actions examples to (1) export changed source .mdx files on merges to main and (2) validate incoming translated PRs (frontmatter presence and basic source/translation structure checks), plus a small fix to the supported Chinese language code entry (zh/zh-Hans).

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