diff --git a/crates/ui-egui/src/i18n/mod.rs b/crates/ui-egui/src/i18n/mod.rs
index e2d8139d6..9698fb82a 100644
--- a/crates/ui-egui/src/i18n/mod.rs
+++ b/crates/ui-egui/src/i18n/mod.rs
@@ -457,19 +457,33 @@ fn detect_system_lang() -> Lang {
     Lang::EN
 }
 
-/// Index into [`LANGUAGES`] of the language the UI is drawn in this frame.
-static CURRENT: AtomicUsize = AtomicUsize::new(0);
+/// Index into [`LANGUAGES`] of the language the UI is drawn in this frame. The app keeps one
+/// index for the whole process. Unit tests keep one per thread, as `ui_mirror!` keeps the
+/// shortcut and workspace mirrors, so a language one test sets does not change the language of
+/// the tests running beside it on other threads.
+fn current_index() -> &'static AtomicUsize {
+    #[cfg(not(test))]
+    {
+        static CURRENT: AtomicUsize = AtomicUsize::new(0);
+        &CURRENT
+    }
+    #[cfg(test)]
+    {
+        thread_local!(static CURRENT: &'static AtomicUsize = Box::leak(Box::default()));
+        CURRENT.with(|c| *c)
+    }
+}
 
 /// Set the UI language for drawing (the shell calls this once per frame from the preference), so
 /// widgets can translate without every call site carrying a language around.
 pub fn set_current(lang: Lang) {
     let i = LANGUAGES.iter().position(|l| l.code == lang.code()).unwrap_or(0);
-    CURRENT.store(i, Ordering::Relaxed);
+    current_index().store(i, Ordering::Relaxed);
 }
 
 /// The language the UI is drawn in.
 pub fn current() -> Lang {
-    Lang(LANGUAGES.get(CURRENT.load(Ordering::Relaxed)).unwrap_or(&LANGUAGES[0]))
+    Lang(LANGUAGES.get(current_index().load(Ordering::Relaxed)).unwrap_or(&LANGUAGES[0]))
 }
 
 /// Does `lang` have a catalog entry for this plain string? (English never does: it is the source.)
diff --git a/crates/ui-egui/src/i18n/tests.rs b/crates/ui-egui/src/i18n/tests.rs
index f5afab545..3ab33db42 100644
--- a/crates/ui-egui/src/i18n/tests.rs
+++ b/crates/ui-egui/src/i18n/tests.rs
@@ -100,6 +100,15 @@ fn current_language_is_a_frame_setting() {
     assert_eq!(t("Layers"), "Layers");
 }
 
+/// Unit tests run on parallel threads, and each one draws in its own language: a language set on
+/// another thread (by a test running beside this one) leaves this thread's text in English.
+#[test]
+fn a_language_set_by_another_test_leaves_this_one_in_english() {
+    std::thread::spawn(|| set_current(ZH())).join().unwrap();
+    assert_eq!(current(), Lang::EN);
+    assert_eq!(t("Layers"), "Layers");
+}
+
 #[test]
 fn lookups_fall_back_to_english() {
     assert_eq!(tr(Lang::EN, "Layers"), "Layers");
@@ -197,8 +206,7 @@ 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: the drawing language is
-/// process-wide, and a Japanese frame would translate the tests running beside this one.
+/// defaults while it is Japanese (#432). The frames stay in English.
 #[test]
 fn the_engine_follows_the_interface_language() {
     let mut app = crate::VectorcraftApp::new(vectorcraft_engine::Session::new(), crate::Services::default());