Simplified Chinese language switch is incomplete on the course website

Problem

Selecting Simplified Chinese (lang=zh) does not consistently translate the course website.

Reproduction

1. Open https://aiengineeringfromscratch.com/?lang=zh
2. Open https://aiengineeringfromscratch.com/learning-paths.html?lang=zh
3. Open https://aiengineeringfromscratch.com/catalog.html?lang=zh
4. Select or enable Simplified Chinese.

Expected behavior

The visible page content should be displayed in Simplified Chinese.

Actual behavior

Most page content remains in English even though:

• The URL contains lang=zh
• The language selector displays Simplified Chinese
• Some lesson pages are partially translated

For example, this lesson contains some Chinese text, but the navigation, sidebar, checkpoints, and other interface text remain in English:

https://aiengineeringfromscratch.com/lesson?path=phases%2F11-llm-engineering%2F01-prompt-engineering&lang=zh&learningPath=applied-ai-engineer

Additional information

The repository documents internationalization here:

https://github.com/rohitg00/ai-engineering-from-scratch/blob/main/docs/i18n.md

The Chinese README exists, but the live website appears to have incomplete Chinese coverage or inconsistent fallback behavior.

Related issue: #404