Tests: Match System resolves to English in ui-egui test builds

Fixes #3244.

The ui-egui tests look up English labels, but Session::default() keeps the
language pref "system", so on a German (or any non-English) system they
read translated labels and fail. Match System now resolves to English under
cfg(test) (lib tests) and under a new test-only `test-support` feature,
which the crate's own dev-dependency on itself turns on for tests/it,
examples and workspace test builds. Product builds are unchanged; tests
about a translation still set their language explicitly. The Match System
test now asserts the English pin explicitly.

Cargo.lock

@@ -1988,6 +1988,7 @@ dependencies = [
  "effectcraft-segment",
  "effectcraft-text",
  "effectcraft-time",
+ "effectcraft-ui-egui",
  "eframe",
  "egui",
  "egui_extras",

crates/ui-egui/Cargo.toml

@@ -45,7 +45,17 @@ craft-update = { workspace = true }
 pollster = { workspace = true }
 sys-locale = { workspace = true }
 
+[features]
+# Tests only, never enable in a product: Settings ▸ General ▸ Language ▸ Match System resolves to
+# English, so English label lookups pass on any system language (#3244). The self dev-dependency
+# below turns it on wherever dev-dependencies are built: this crate's integration tests, its
+# examples (`snapshot` shots are deterministic English unless a step sets a language) and
+# workspace test builds of the apps (`cargo test --workspace`).
+test-support = []
+
 [dev-dependencies]
+# This crate's own integration tests build it with `test-support` (see `[features]`).
+effectcraft-ui-egui = { workspace = true, features = ["test-support"] }
 # Fonts for kittest: the workspace `egui` features are what test roots resolve, but stating them
 # here documents the requirement (kittest's own dev-dependencies do not propagate to consumers).
 egui = { workspace = true, features = ["default_fonts", "monochrome_emoji_fonts"] }

crates/ui-egui/src/i18n.rs

@@ -84,7 +84,14 @@ pub(crate) fn swedish(app: &EffectcraftApp) -> bool {
 /// Settings ▸ General ▸ Language ▸ Match System: the operating system's interface language
 /// where EffectCraft has it, else English (#229), as After Effects installs in the system's
 /// language. The browser build follows the locales its page passes in ([`set_system_locales`]).
+///
+/// Test builds (this crate's unit tests, and its integration tests through the `test-support`
+/// feature) resolve Match System to English, so tests that look up English labels pass on any
+/// system language (#3244); tests about a translation choose their language explicitly.
 fn system_language() -> &'static str {
+    if cfg!(any(test, feature = "test-support")) {
+        return "en";
+    }
     #[cfg(not(target_arch = "wasm32"))]
     {
         static LANGUAGE: std::sync::OnceLock<&'static str> = std::sync::OnceLock::new();
@@ -9169,13 +9176,22 @@ mod tests {
         }
         let mut app = EffectcraftApp::new(effectcraft_engine::Session::default());
         assert_eq!(app.session.prefs.general.language, "system", "the default");
-        assert_eq!(japanese(&app), system_language() == "ja");
-        assert_eq!(simplified(&app), system_language() == "zh-hans");
-        assert_eq!(traditional(&app), system_language() == "zh-hant");
-        assert_eq!(spanish(&app), system_language() == "es");
-        assert_eq!(czech(&app), system_language() == "cs");
-        assert_eq!(polish(&app), system_language() == "pl");
-        assert_eq!(swedish(&app), system_language() == "sv");
+        assert_eq!(system_language(), "en", "test builds pin Match System to English (#3244)");
+        assert_eq!(language(&app), "en");
+        for (flag, name) in [
+            (japanese(&app), "ja"),
+            (simplified(&app), "zh-hans"),
+            (traditional(&app), "zh-hant"),
+            (ukrainian(&app), "uk"),
+            (spanish(&app), "es"),
+            (brazilian_portuguese(&app), "pt-br"),
+            (czech(&app), "cs"),
+            (polish(&app), "pl"),
+            (georgian(&app), "ka"),
+            (swedish(&app), "sv"),
+        ] {
+            assert!(!flag, "Match System shows {name} in a test build");
+        }
         app.session.execute("prefs.set", json!({"key":"general.language", "value":"ja"})).unwrap();
         assert!(japanese(&app));
         app.session.execute("prefs.set", json!({"key":"general.language", "value":"zh-hans"})).unwrap();