feat: internationalization — multi-language content support

[perasperaactual]

Problem

No internationalization support exists. Sites that need multi-language content must implement it entirely in custom React code.

Strategic review reference

From ROADMAP.md (Medium-term: Framework):

Internationalization — Multi-language content support via per-locale content directories or inline locale maps in YAML.

Design considerations

• Per-locale content directories (e.g., content/en/pages/, content/es/pages/) vs. inline locale maps in YAML
• Locale-aware routing (Next.js i18n routing integration)
• Site config locale settings
• Impact on prebuild pipeline, static generation helpers

Notes

This is a longer-term item. Design should be scoped before implementation begins.