Tests: the engine-language test runs a Japanese frame again

the_engine_follows_the_interface_language drew only English frames
after 6efe9df3e, because a Japanese frame set the drawing language for
every test running beside it. It set the engine's language by hand and
never checked that a Japanese frame hands the engine ja. Since #2570
each unit-test thread has its own drawing language.

The test again runs a frame with the interface preference set to
Japanese and checks that the engine gets ja, then runs a frame with
the preference back on auto and checks that it gets en. It also checks
that a thread started after the Japanese frame still draws in English.
With the drawing language process-wide, as before #2570, the test
fails, and the zoom hint test beside it showed its hint in Japanese in
12 of 200 runs.

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

@@ -209,7 +209,8 @@ fn partial_catalogs_only_translate_strings_the_ui_shows() {
 }
 
 /// The engine learns the language the UI is drawn in each frame: new type takes the Japanese
-/// defaults while it is Japanese (#432). The frames stay in English.
+/// defaults while it is Japanese (#432). A Japanese frame draws in Japanese on this test's thread
+/// only: the tests running beside it on other threads still draw in English.
 #[test]
 fn the_engine_follows_the_interface_language() {
     let mut app = crate::VectorcraftApp::new(vectorcraft_engine::Session::new(), crate::Services::default());
@@ -220,14 +221,19 @@ fn the_engine_follows_the_interface_language() {
     frame(&mut app);
     assert_eq!(app.session.ui_language.as_deref(), Some("en"), "tests resolve `auto` to English");
     assert!(!app.session.japanese_interface());
-    // Each frame hands the engine the language it draws in, whatever the engine had.
-    app.session.ui_language = Some("ja".into());
+    app.session.prefs.interface_language = "ja".into();
+    frame(&mut app);
+    assert_eq!(app.session.ui_language.as_deref(), Some("ja"));
     assert!(app.session.japanese_interface());
+    assert_eq!(current().code(), "ja");
+    // A test running beside this one on another thread still draws in English.
+    assert_eq!(std::thread::spawn(|| (current(), t("Layers"))).join().unwrap(), (Lang::EN, "Layers"));
+    // With the preference back on `auto`, the next frame hands the engine `en` again.
+    app.session.prefs.interface_language = "auto".into();
     frame(&mut app);
     assert_eq!(app.session.ui_language.as_deref(), Some("en"));
-    // The language a frame would draw in follows the preference.
-    app.session.prefs.interface_language = "ja".into();
-    assert_eq!(app.ui_language().code(), "ja");
+    assert!(!app.session.japanese_interface());
+    assert_eq!(current(), Lang::EN);
 }
 
 /// `app.language` sets the `interfaceLanguage` preference, which is what persists; the checked