diff --git a/README.md b/README.md
index 1a6aa0c..7e83b0b 100644
--- a/README.md
+++ b/README.md
@@ -133,7 +133,14 @@ platform), dispatch the workflow with `platforms: android` or `platforms: ios`.
 ## The gn args are load-bearing
 
 They are commented at their definition in `scripts/matrix/build-android.sh` and
-`scripts/matrix/build-ios.sh`. Three worth knowing:
+`scripts/matrix/build-ios.sh`. Four worth knowing:
+
+- `v8_enable_i18n_support=true` with `icu_use_data_file=false` — the 10.3
+  builds had i18n off, so `Intl` did not exist and `localeCompare`,
+  `toLocale*String`, `String#normalize` and RegExp `\p{…}` were missing or
+  locale-blind. The ICU data is linked into the libraries (Chromium's `android`
+  filter on Android, `ios` on iOS and Catalyst, `common` on tvOS), so
+  `V8::InitializeICU` is a no-op and the runtimes need no `icudtl.dat`.
 
 - `cppgc_enable_caged_heap=false` (iOS only) — it defaults to **true on arm64**
   and, contrary to how the flags read, does *not* track
diff --git a/scripts/matrix/build-android.sh b/scripts/matrix/build-android.sh
index 45a3317..388758c 100755
--- a/scripts/matrix/build-android.sh
+++ b/scripts/matrix/build-android.sh
@@ -68,9 +68,16 @@ if [ "$(uname)" != "Linux" ] && { [ "$CPU" = "arm" ] || [ "$CPU" = "x86" ]; }; t
     exit 1
 fi
 
-# Reproduces the shape of the 10.3 build these libraries replace: JIT and
-# WebAssembly on, i18n off. Every deviation from the gn defaults is deliberate:
+# Reproduces the shape of the 10.3 build these libraries replace (JIT and
+# WebAssembly on), except that i18n is now on. Every deviation from the gn
+# defaults is deliberate:
 #
+#   v8_enable_i18n_support / icu_use_data_file
+#       10.3 shipped with i18n off, which removes Intl, the locale-aware
+#       toLocale*String/localeCompare/case mapping, String#normalize and RegExp
+#       \p{...}. The ICU data (Chromium's android filter) is linked into the
+#       monolith rather than loaded from icudtl.dat, so V8::InitializeICU is a
+#       no-op and the runtime needs no data file to find.
 #   v8_array_buffer_internal_field_count / ..._view_...
 #       Defaulted to 2 in 10.3, default to 0 in 14.9. The Android runtime
 #       marshals an ArrayBuffer to a Java NIO buffer by linking its
@@ -113,7 +120,7 @@ GN_ARGS="
     v8_static_library=true
     v8_use_external_startup_data=false
 
-    v8_enable_i18n_support=false
+    v8_enable_i18n_support=true
     v8_enable_webassembly=true
     v8_enable_sandbox=false
     v8_enable_temporal_support=false
diff --git a/scripts/matrix/build-ios.sh b/scripts/matrix/build-ios.sh
index 19e832a..9aa3180 100755
--- a/scripts/matrix/build-ios.sh
+++ b/scripts/matrix/build-ios.sh
@@ -98,6 +98,12 @@ MODULES=(
 # process by device RAM, so that reservation fails outright on smaller devices
 # -- four tries, then Oilpan's fatal OOM handler, before any JS runs. It crashed
 # on an iPad Air while an iPad Pro was fine.
+#
+# v8_enable_i18n_support=true with icu_use_data_file=false links the ICU data
+# into the icudata object, which libv8_third_party picks up with the rest of
+# ICU, so V8::InitializeICU is a no-op and the runtime needs no icudtl.dat.
+# iOS and Catalyst get Chromium's ios data filter; tvOS gets the common one,
+# because the ICU build only selects the ios filter when use_blink is false.
 GN_ARGS="
     target_os=\"ios\"
     treat_warnings_as_errors=false
@@ -113,7 +119,7 @@ GN_ARGS="
     v8_use_external_startup_data=false
     v8_enable_sandbox=false
     v8_enable_debugging_features=false
-    v8_enable_i18n_support=false
+    v8_enable_i18n_support=true
     v8_enable_temporal_support=false
     v8_enable_pointer_compression=false
     v8_enable_v8_checks=false
@@ -201,10 +207,10 @@ archive_lib "libcrdtp_platform" "$OUTFOLDER/obj/third_party/inspector_protocol/c
 archive_lib "libinspector" "$OUTFOLDER/obj/src/inspector/inspector/*.o"
 archive_lib "libinspector_string_conversions" "$OUTFOLDER/obj/src/inspector/inspector_string_conversions/*.o"
 
-# abseil, simdutf and highway plus the libm trig sources: referenced by v8_base
-# but part of no module above. zlib and inspector_protocol are excluded because
-# they ship as libzip/libcrdtp; rust is excluded because disabling Temporal
-# leaves only stale objects behind.
+# ICU (code and data), abseil, simdutf and highway plus the libm trig sources:
+# referenced by v8_base but part of no module above. zlib and inspector_protocol
+# are excluded because they ship as libzip/libcrdtp; rust is excluded because
+# disabling Temporal leaves only stale objects behind.
 THIRD_PARTY_OBJS=$(find "$OUTFOLDER/obj/third_party" -name "*.o" \
     ! -path "*/zlib/*" ! -path "*/inspector_protocol/*" ! -path "*/rust/*" 2>/dev/null)
 archive_lib "libv8_third_party" "$THIRD_PARTY_OBJS $OUTFOLDER/obj/libm/*.o"
diff --git a/tests/apple-args.py b/tests/apple-args.py
index fcd1590..1ed7e81 100644
--- a/tests/apple-args.py
+++ b/tests/apple-args.py
@@ -35,5 +35,11 @@ def test_ios_and_catalyst_do_not_receive_tvos_overrides(self):
             self.assertIn('is_debug=false', args)
             self.assertEqual('v8_enable_lite_mode=true' in args, 'catalyst' not in variant)
 
+    def test_every_variant_embeds_icu(self):
+        for variant in ['arm64-device', 'arm64-simulator', 'x64-simulator', 'arm64-catalyst', 'x64-catalyst', 'arm64-tvdevice', 'arm64-tvsimulator']:
+            args = self.args_for(variant)
+            self.assertIn('v8_enable_i18n_support=true', args)
+            self.assertIn('icu_use_data_file=false', args)
+
 if __name__ == '__main__':
     unittest.main()