Enhancement: add Chinese (zh-CN) localization for diagram labels

[chu0119]

Feature request

Mermaid supports multiple languages but currently lacks Chinese (zh-CN) localization for built-in diagram labels and error messages. Adding zh-CN support would improve accessibility for the large Chinese-speaking developer community.

[konradkelly]

Hi, I'm interested in picking up #7757. I was thinking of starting small: a locale config (en / zh-CN) and translating the main error paths first (the error SVG, Langium parse messages, unknown diagram type). Stuff like JISON parser errors and diagram field labels could come later. Want to make sure that's a reasonable first PR before I dive in.

[konradkelly]

Opened #8044 with a first pass. It came out narrower than what I proposed above.

Mermaid has fewer hardcoded English strings than I expected; almost all diagram text is user-supplied. The real candidates are the syntax error diagram, Cynefin domain labels, the Gantt date axis (d3's month/weekday names), and the Requirement diagram's type/risk/verification labels. Since there's no i18n infrastructure today, I kept the PR to the smallest thing that establishes a pattern: a locale config option, a catalog with English as the typed source of truth, and the error diagram as the only consumer. The rest are self-contained follow-ups.

One deliberate change: thrown Error messages stay English. UnknownDiagramError and MermaidParseError are developer-facing, asserted in tests and pasted into bug reports and search engines, so translating them costs debuggability without helping the people this issue is about. MermaidParseError's inner text also comes from Chevrotain, so doing it properly needs a per-grammar errorMessageProvider. Happy to revisit.