M14: let neutral Windows locales fall through to display language

Co-authored-by: Codex <codex@openai.com>

ROADMAP.md

@@ -139,6 +139,8 @@ M0 → M1 → M2 → M3 → M4 must happen in order. After M4, M5–M12 can run
 
 Newest first. One line per session: the date, what moved, and the new overall percentage.
 
+- **2026-10-09 (M14, Windows Auto):** Neutral launcher locales (`C`, `C.UTF-8`, `POSIX`) no longer select English ahead of the Windows preferred display-language list. Explicit supported locale overrides retain their priority. Regression tests use existing catalogs and cover neutral/mixed overrides, unsupported first display tags and multiple preferred Windows languages. Catalogs and other platforms are unchanged; overall effort estimate remains about 30–35%.
+
 - **2026-10-09 (community, M14, German):** Deutsch (`de`) is a complete interface catalog (2,035 entries) covering every command, All tools entry, dialog, panel and notice, selected automatically for `de`, `de_DE`, `de_AT`, `de_CH` and other German locales, and from Preferences. Informal "du", infinitive for commands. Coverage tests check commands, All tools labels and every `tl!` literal; history labels keep captured names; the bundled Latin faces cover umlauts, ß and German quotes. Overall estimate unchanged (about 30–35%).
 - **2026-10-09 (community, M7, Arabic added text):** Add text writes Arabic into the PDF instead of `?`. The text is shaped with the craft-fonts `Arab` face through harfrust (already in the tree via egui): joining forms, ligatures and mark positions. Each line is laid out in display order by the Unicode bidirectional algorithm, so Arabic mixed with Latin words, European or Arabic-Indic digits and brackets reads correctly; justified right-to-left paragraphs end at the right edge. Each cluster (a letter with its dots or marks) becomes one glyph of a Type 3 font built from the face's outlines, with a ToUnicode entry, so `text_extract`, copy and search get every letter once and no font file is embedded. Wrapping measures shaped words. Each item has its own fonts under names free on the page (a full save renumbers objects but not resource names); updating or deleting the item removes them. Without the face, adding Arabic text returns a clear error and changes nothing, while an existing item holding Arabic still moves and retypes, drawn as `?` as before. Tests cover shaping, display order, ToUnicode, font names and clean-up on one page, and odd input (empty lines, direction controls, lone marks, 18,000 characters). Not yet: bold/italic Arabic, Hebrew, right-to-left typing in the box (misc.rtl-ui), editing existing Arabic text (edit.rtl-cjk-editing). The craft-fonts revision pinned for releases predates its Arabic face, so release builds report the missing font until the pin moves. Overall estimate unchanged (about 30–35%).
 - **2026-10-09 (M2, predictor hardening):** Empty PNG predictor frames return without row buffers. Packed TIFF Predictor 2 rows reject overflowing dimensions and size per-component scratch to the samples present in the input, aligned with upstream lopdf#589. Synthetic regressions cover malformed dimensions, partial rows, 1/2/4-bit decoding and padding. Overall estimate unchanged (about 30–35%).

crates/ui-egui/src/i18n/mod.rs

@@ -235,7 +235,7 @@ pub fn system_lang() -> Lang {
 #[cfg(not(target_arch = "wasm32"))]
 fn detect_system_lang() -> Lang {
     for var in ["LC_ALL", "LC_MESSAGES", "LANG"] {
-        if let Some(l) = std::env::var(var).ok().filter(|v| !v.is_empty()).and_then(|v| lang_from_tag(&v)) {
+        if let Some(l) = std::env::var(var).ok().and_then(|v| language_override(&v)) {
             return l;
         }
     }
@@ -255,6 +255,17 @@ fn detect_system_lang() -> Lang {
     Lang::EN
 }
 
+/// A named locale can override the display language. On Windows, the neutral
+/// C/POSIX locale inherited from a launcher says nothing about the user's UI language.
+#[cfg(not(target_arch = "wasm32"))]
+fn language_override(tag: &str) -> Option<Lang> {
+    let tag = tag.trim();
+    if cfg!(target_os = "windows") && matches!(candidates(tag).first().map(String::as_str), Some("c" | "posix")) {
+        return None;
+    }
+    lang_from_tag(tag)
+}
+
 /// The Windows display languages in preference order, one tag per line, queried once when Auto
 /// first resolves. `sys-locale` asks Windows directly (`GetUserPreferredUILanguages`), so no
 /// process is started.
@@ -453,6 +464,20 @@ mod tests {
         assert_eq!(first_supported("\r\n"), None);
     }
 
+    #[cfg(target_os = "windows")]
+    #[test]
+    fn neutral_locale_does_not_override_display_language() {
+        let fr = Lang::from_code("fr").unwrap();
+        for tag in ["C", "C.UTF-8", "POSIX", "posix.UTF-8", "", "  C.UTF-8  ", "unsupported"] {
+            assert_eq!(language_override(tag), None, "{tag}");
+            assert_eq!(language_override(tag).or_else(|| first_supported("unsupported-XY\nfr-FR\nen-US")), Some(fr));
+        }
+        assert_eq!(["C.UTF-8", "POSIX", "fr_FR.UTF-8"].into_iter().find_map(language_override), Some(fr));
+        assert_eq!(language_override("fr_FR.UTF-8"), Some(fr));
+        assert_eq!(language_override("en_US.UTF-8"), Some(Lang::EN));
+        assert_eq!(language_override("ja_JP.UTF-8"), Some(JA()));
+    }
+
     #[cfg(target_os = "windows")]
     #[test]
     fn windows_display_language_query_returns_a_locale_tag() {

docs/localization.md

@@ -2,7 +2,7 @@
 
 Choose **Menu > Edit > Preferences…** (Command-comma on macOS, Ctrl-comma elsewhere) **> Interface language** and select **Auto**, **English**, **日本語**, **简体中文**, **繁體中文**, **Русский**, **Български**, **Čeština**, **Português (Brasil)**, **Deutsch**, **Español**, **Français** or **తెలుగు**. The change applies immediately and persists between launches. Command ids, document contents and file names are unchanged.
 
-**Auto** (the default) follows the system language: `LC_ALL`, `LC_MESSAGES` or `LANG`, then the preferred-languages list on macOS. Any Portuguese locale (`pt_BR`, `pt_PT`) uses the Brazilian catalog, any French locale (`fr`, `fr_FR`, `fr_CA`, `fr_BE`) the French one, any German locale (`de`, `de_DE`, `de_AT`, `de_CH`) the German one, and Chinese locales for Taiwan, Hong Kong and Macau (`zh_TW`, `zh_HK`, `zh_MO`, `zh-Hant`) the Traditional Chinese one; other Chinese locales (`zh`, `zh_CN`, `zh_SG`, `zh-Hans`) the Simplified Chinese one. Telugu locales (`te`, `te_IN`) use the Telugu catalog, and Bulgarian locales (`bg`, `bg_BG`) the Bulgarian one. A system language without a catalog shows English. On Windows, **Auto** uses the Windows display languages, in preference order, when no supported locale is selected by those environment variables. The query asks Windows directly (`GetUserPreferredUILanguages`, through the `sys-locale` crate) without starting a process, and is cached for the process. If detection fails or the display language has no catalog, PdfCraft shows English.
+**Auto** (the default) follows the system language: `LC_ALL`, `LC_MESSAGES` or `LANG`, then the preferred-languages list on macOS. Any Portuguese locale (`pt_BR`, `pt_PT`) uses the Brazilian catalog, any French locale (`fr`, `fr_FR`, `fr_CA`, `fr_BE`) the French one, any German locale (`de`, `de_DE`, `de_AT`, `de_CH`) the German one, and Chinese locales for Taiwan, Hong Kong and Macau (`zh_TW`, `zh_HK`, `zh_MO`, `zh-Hant`) the Traditional Chinese one; other Chinese locales (`zh`, `zh_CN`, `zh_SG`, `zh-Hans`) the Simplified Chinese one. Telugu locales (`te`, `te_IN`) use the Telugu catalog, and Bulgarian locales (`bg`, `bg_BG`) the Bulgarian one. A system language without a catalog shows English. On Windows, **Auto** uses the Windows display languages, in preference order, when no supported locale is selected by those environment variables. Neutral `C`/`POSIX` locale values (including `C.UTF-8`) inherited from a launcher do not override the Windows display language; explicitly named supported locales such as `fr_FR.UTF-8` still do. The query asks Windows directly (`GetUserPreferredUILanguages`, through the `sys-locale` crate) without starting a process, and is cached for the process. If detection fails or the display language has no catalog, PdfCraft shows English.
 
 The control channel exposes the setting through `ui.set`:
 

parity/acrobat-features.toml

@@ -9818,8 +9818,8 @@ milestone = "M14"
 status = "partial"
 commands = ["app.preferences"]
 tools = []
-evidence = ["crates/ui-egui/src/i18n/mod.rs::windows_display_language_resolves_chinese_scripts", "crates/ui-egui/src/i18n/mod.rs::windows_display_language_query_returns_a_locale_tag", "crates/ui-egui/src/i18n/mod.rs::language_persists_and_invalid_input_keeps_current_language", "crates/ui-egui/src/i18n/mod.rs::bundled_catalogs_are_consistent", "crates/ui-egui/src/i18n/mod.rs::japanese_covers_commands_and_catalogue", "crates/ui-egui/src/i18n/mod.rs::simplified_chinese_covers_commands_and_catalogue", "crates/ui-egui/src/i18n/mod.rs::simplified_chinese_covers_ui_literals", "crates/ui-egui/src/i18n/mod.rs::simplified_chinese_history_and_diagnostics_preserve_user_values", "crates/ui-egui/tests/control.rs::simplified_chinese_signature_prompts_and_errors_keep_document_state", "crates/ui-egui/tests/control.rs::language_switch_preserves_document_and_command_ids", "crates/ui-egui/tests/control.rs::japanese_controls_and_search_keep_command_ids", "crates/ui-egui/tests/control.rs::japanese_dialogs_errors_and_custom_action_names", "crates/ui-egui/src/i18n/mod.rs::french_is_registered", "crates/ui-egui/src/i18n/mod.rs::french_covers_commands_and_catalogue", "crates/ui-egui/src/i18n/mod.rs::french_covers_ui_literals", "crates/ui-egui/src/i18n/mod.rs::french_history_and_diagnostics_preserve_user_values", "crates/ui-egui/src/i18n/mod.rs::telugu_covers_commands_and_catalogue", "crates/ui-egui/src/i18n/mod.rs::telugu_labels_have_glyphs"]
-notes = "Preferences switches Auto (system language)/English/Japanese/Simplified Chinese/Traditional Chinese/Czech/Brazilian Portuguese/German/Spanish/French/Telugu and persists; ui.set language auto|en|ja|zh-hans|zh-hant|cs|pt-br|de|es|fr|te and ui.state expose it. TSV catalogs per language (same system as PhotoCraft); every dialog, panel and notice goes through tl!. Japanese, Traditional Chinese, Simplified Chinese, German, Spanish and French cover commands, dialogs, panels, shortcuts and history labels; Czech and Brazilian Portuguese the menus. French locales (fr, fr_FR, fr_CA, fr_BE) select the French catalog, German locales (de, de_DE, de_AT, de_CH) the German one. Engine/OS error details stay untranslated. Auto detects the Windows display language after locale environment overrides, with Chinese script/region mapping and English fallback. Missing: Czech and Brazilian Portuguese dialogs, Chinese and Telugu font delivery in releases, a full localization audit."
+evidence = ["crates/ui-egui/src/i18n/mod.rs::windows_display_language_resolves_chinese_scripts", "crates/ui-egui/src/i18n/mod.rs::neutral_locale_does_not_override_display_language", "crates/ui-egui/src/i18n/mod.rs::windows_display_language_query_returns_a_locale_tag", "crates/ui-egui/src/i18n/mod.rs::language_persists_and_invalid_input_keeps_current_language", "crates/ui-egui/src/i18n/mod.rs::bundled_catalogs_are_consistent", "crates/ui-egui/src/i18n/mod.rs::japanese_covers_commands_and_catalogue", "crates/ui-egui/src/i18n/mod.rs::simplified_chinese_covers_commands_and_catalogue", "crates/ui-egui/src/i18n/mod.rs::simplified_chinese_covers_ui_literals", "crates/ui-egui/src/i18n/mod.rs::simplified_chinese_history_and_diagnostics_preserve_user_values", "crates/ui-egui/tests/control.rs::simplified_chinese_signature_prompts_and_errors_keep_document_state", "crates/ui-egui/tests/control.rs::language_switch_preserves_document_and_command_ids", "crates/ui-egui/tests/control.rs::japanese_controls_and_search_keep_command_ids", "crates/ui-egui/tests/control.rs::japanese_dialogs_errors_and_custom_action_names", "crates/ui-egui/src/i18n/mod.rs::french_is_registered", "crates/ui-egui/src/i18n/mod.rs::french_covers_commands_and_catalogue", "crates/ui-egui/src/i18n/mod.rs::french_covers_ui_literals", "crates/ui-egui/src/i18n/mod.rs::french_history_and_diagnostics_preserve_user_values", "crates/ui-egui/src/i18n/mod.rs::telugu_covers_commands_and_catalogue", "crates/ui-egui/src/i18n/mod.rs::telugu_labels_have_glyphs"]
+notes = "Preferences switches Auto (system language)/English/Japanese/Simplified Chinese/Traditional Chinese/Czech/Brazilian Portuguese/German/Spanish/French/Telugu and persists; ui.set language auto|en|ja|zh-hans|zh-hant|cs|pt-br|de|es|fr|te and ui.state expose it. TSV catalogs per language (same system as PhotoCraft); every dialog, panel and notice goes through tl!. Japanese, Traditional Chinese, Simplified Chinese, German, Spanish and French cover commands, dialogs, panels, shortcuts and history labels; Czech and Brazilian Portuguese the menus. French locales (fr, fr_FR, fr_CA, fr_BE) select the French catalog, German locales (de, de_DE, de_AT, de_CH) the German one. Engine/OS error details stay untranslated. Auto detects the Windows display language after explicitly named supported locale overrides; neutral C/POSIX launcher locales are skipped on Windows, with Chinese script/region mapping and English fallback. Missing: Czech and Brazilian Portuguese dialogs, Chinese and Telugu font delivery in releases, a full localization audit."
 
 [[feature]]
 id = "misc.prefs-reading"