[BUG] Japanese text renders with Simplified Chinese glyphs (Han unification issue) in Claude Desktop for macOS

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report

What's Wrong?

After a recent update, Japanese text in the chat UI of Claude Desktop for macOS started rendering with Simplified Chinese glyphs (PingFang SC-style) instead of Japanese glyphs. This is the classic Han unification problem: characters such as 直, 骨, 誤, 令 are displayed with their zh-CN glyph variants, which looks clearly wrong to Japanese readers.

Notably, no Chinese language is configured anywhere on this system -- the macOS preferred languages list contains only English and Japanese. The renderer nevertheless selects Simplified Chinese glyph variants for unified CJK code points, which means the CJK font fallback is not consulting the OS language preferences at all.

Environment

• App: Claude Desktop for macOS, version 1.20186.0
• macOS: 26.5.2 (Apple Silicon)
• macOS preferred languages: English (primary), Japanese (secondary) -- no Chinese configured
• Claude UI language: English

Reproduction / Isolation

• Chat font setting = Default -> Japanese text renders with SC glyphs
• Chat font setting = Match system -> renders correctly (Hiragino Sans, correct Japanese glyphs)

This suggests the default font stack (or the effective lang attribute on chat content) resolves CJK fallback from the app locale alone (en), rather than from either the content language or the OS preferred-languages list. When the locale is en, the renderer appears to fall through to zh-CN font preferences for unified CJK code points.

Expected Behavior

Japanese text should render with Japanese glyphs. In particular, when the OS preferred-languages list includes Japanese and contains no Chinese, there is no user setting that justifies selecting SC glyph variants. Possible fixes:

• Respect the macOS preferred-languages order for CJK font fallback (this is what Chromium normally derives its --accept-lang fallback from), or
• Set lang="ja" on Japanese content, or
• Order the default font stack so that a Japanese font (Hiragino Sans) precedes any SC/TC fonts in CJK fallback.

Workaround

Setting chat font to "Match system" resolves the issue.