diff --git a/Libraries/LibJS/Runtime/Intl/DurationFormat.cpp b/Libraries/LibJS/Runtime/Intl/DurationFormat.cpp
index bd760204426..f5215499359 100644
--- a/Libraries/LibJS/Runtime/Intl/DurationFormat.cpp
+++ b/Libraries/LibJS/Runtime/Intl/DurationFormat.cpp
@@ -407,7 +407,7 @@ Vector<DurationFormatPart> format_numeric_hours(VM& vm, DurationFormat const& du
 
     for (auto& part : hours_parts) {
         // a. Append the Record { [[Type]]: part.[[Type]], [[Value]]: part.[[Value]], [[Unit]]: "hour" } to result.
-        result.unchecked_append({ .type = part.type, .value = move(part.value), .unit = "hour"sv });
+        result.unchecked_append({ .type = part.type, .value = part.value.to_utf8_but_should_be_ported_to_utf16(), .unit = "hour"sv });
     }
 
     // 13. Return result.
@@ -472,7 +472,7 @@ Vector<DurationFormatPart> format_numeric_minutes(VM& vm, DurationFormat const&
 
     for (auto& part : minutes_parts) {
         // a. Append the Record { [[Type]]: part.[[Type]], [[Value]]: part.[[Value]], [[Unit]]: "minute" } to result.
-        result.unchecked_append({ .type = part.type, .value = move(part.value), .unit = "minute"sv });
+        result.unchecked_append({ .type = part.type, .value = part.value.to_utf8_but_should_be_ported_to_utf16(), .unit = "minute"sv });
     }
 
     // 14. Return result.
@@ -560,7 +560,7 @@ Vector<DurationFormatPart> format_numeric_seconds(VM& vm, DurationFormat const&
 
     for (auto& part : seconds_parts) {
         // a. Append the Record { [[Type]]: part.[[Type]], [[Value]]: part.[[Value]], [[Unit]]: "second" } to result.
-        result.unchecked_append({ .type = part.type, .value = move(part.value), .unit = "second"sv });
+        result.unchecked_append({ .type = part.type, .value = part.value.to_utf8_but_should_be_ported_to_utf16(), .unit = "second"sv });
     }
 
     // 18. Return result.
@@ -922,7 +922,7 @@ Vector<DurationFormatPart> partition_duration_format_pattern(VM& vm, DurationFor
 
                 for (auto& part : parts) {
                     // a. Append the Record { [[Type]]: part.[[Type]], [[Value]]: part.[[Value]], [[Unit]]: numberFormatUnit } to list.
-                    list.unchecked_append({ .type = part.type, .value = move(part.value), .unit = number_format_unit.as_string() });
+                    list.unchecked_append({ .type = part.type, .value = part.value.to_utf8_but_should_be_ported_to_utf16(), .unit = number_format_unit.as_string() });
                 }
 
                 // 11. Append list to result.
diff --git a/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp b/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp
index 27213a19d0e..16af55adec0 100644
--- a/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp
+++ b/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp
@@ -152,7 +152,7 @@ Vector<Unicode::NumberFormat::Partition> partition_number_pattern(NumberFormat c
 }
 
 // 16.5.6 FormatNumeric ( numberFormat, x ), https://tc39.es/ecma402/#sec-formatnumber
-String format_numeric(NumberFormat const& number_format, MathematicalValue const& number)
+Utf16String format_numeric(NumberFormat const& number_format, MathematicalValue const& number)
 {
     // 1. Let parts be ? PartitionNumberPattern(numberFormat, x).
     // 2. Let result be the empty String.
@@ -258,7 +258,7 @@ ThrowCompletionOr<Vector<Unicode::NumberFormat::Partition>> partition_number_ran
 }
 
 // 16.5.22 FormatNumericRange ( numberFormat, x, y ), https://tc39.es/ecma402/#sec-formatnumericrange
-ThrowCompletionOr<String> format_numeric_range(VM& vm, NumberFormat const& number_format, MathematicalValue const& start, MathematicalValue const& end)
+ThrowCompletionOr<Utf16String> format_numeric_range(VM& vm, NumberFormat const& number_format, MathematicalValue const& start, MathematicalValue const& end)
 {
     // 1. Let parts be ? PartitionNumberRangePattern(numberFormat, x, y).
     {
diff --git a/Libraries/LibJS/Runtime/Intl/NumberFormat.h b/Libraries/LibJS/Runtime/Intl/NumberFormat.h
index fad9379fca3..e88ddcf2ae6 100644
--- a/Libraries/LibJS/Runtime/Intl/NumberFormat.h
+++ b/Libraries/LibJS/Runtime/Intl/NumberFormat.h
@@ -182,11 +182,11 @@ private:
 
 int currency_digits(StringView currency);
 Vector<Unicode::NumberFormat::Partition> partition_number_pattern(NumberFormat const&, MathematicalValue const& number);
-String format_numeric(NumberFormat const&, MathematicalValue const& number);
+Utf16String format_numeric(NumberFormat const&, MathematicalValue const& number);
 GC::Ref<Array> format_numeric_to_parts(VM&, NumberFormat const&, MathematicalValue const& number);
 ThrowCompletionOr<MathematicalValue> to_intl_mathematical_value(VM&, Value value);
 ThrowCompletionOr<Vector<Unicode::NumberFormat::Partition>> partition_number_range_pattern(VM&, NumberFormat const&, MathematicalValue const& start, MathematicalValue const& end);
-ThrowCompletionOr<String> format_numeric_range(VM&, NumberFormat const&, MathematicalValue const& start, MathematicalValue const& end);
+ThrowCompletionOr<Utf16String> format_numeric_range(VM&, NumberFormat const&, MathematicalValue const& start, MathematicalValue const& end);
 ThrowCompletionOr<GC::Ref<Array>> format_numeric_range_to_parts(VM&, NumberFormat const&, MathematicalValue const& start, MathematicalValue const& end);
 
 }
diff --git a/Libraries/LibUnicode/NumberFormat.cpp b/Libraries/LibUnicode/NumberFormat.cpp
index 7d9e786ad96..35a79a7de60 100644
--- a/Libraries/LibUnicode/NumberFormat.cpp
+++ b/Libraries/LibUnicode/NumberFormat.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2024, Tim Flynn <trflynn89@ladybird.org>
+ * Copyright (c) 2021-2025, Tim Flynn <trflynn89@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -601,7 +601,7 @@ public:
 
     virtual ~NumberFormatImpl() override = default;
 
-    virtual String format(Value const& value) const override
+    virtual Utf16String format(Value const& value) const override
     {
         UErrorCode status = U_ZERO_ERROR;
 
@@ -613,7 +613,7 @@ public:
         if (icu_failure(status))
             return {};
 
-        return icu_string_to_string(result);
+        return icu_string_to_utf16_string(result);
     }
 
     virtual Vector<Partition> format_to_parts(Value const& value) const override
@@ -625,7 +625,7 @@ public:
         return format_to_parts_impl(formatted, value, value);
     }
 
-    virtual String format_range(Value const& start, Value const& end) const override
+    virtual Utf16String format_range(Value const& start, Value const& end) const override
     {
         UErrorCode status = U_ZERO_ERROR;
 
@@ -637,7 +637,7 @@ public:
         if (icu_failure(status))
             return {};
 
-        return icu_string_to_string(result);
+        return icu_string_to_utf16_string(result);
     }
 
     virtual Vector<Partition> format_range_to_parts(Value const& start, Value const& end) const override
@@ -830,7 +830,7 @@ private:
             auto value = formatted_number.tempSubStringBetween(range.start, range.end);
 
             Partition partition;
-            partition.value = icu_string_to_string(value);
+            partition.value = icu_string_to_utf16_string(value);
             apply_to_partition(partition, range.field, range.start);
 
             result.unchecked_append(move(partition));
diff --git a/Libraries/LibUnicode/NumberFormat.h b/Libraries/LibUnicode/NumberFormat.h
index 586b3f238d8..6b4699deb72 100644
--- a/Libraries/LibUnicode/NumberFormat.h
+++ b/Libraries/LibUnicode/NumberFormat.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2024, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2021-2025, Tim Flynn <trflynn89@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -9,6 +9,7 @@
 #include <AK/Optional.h>
 #include <AK/String.h>
 #include <AK/StringView.h>
+#include <AK/Utf16String.h>
 #include <AK/Variant.h>
 #include <AK/Vector.h>
 #include <LibUnicode/Forward.h>
@@ -149,16 +150,16 @@ public:
 
     struct Partition {
         StringView type;
-        String value;
+        Utf16String value;
         StringView source;
     };
 
     using Value = Variant<double, String>;
 
-    virtual String format(Value const&) const = 0;
+    virtual Utf16String format(Value const&) const = 0;
     virtual Vector<Partition> format_to_parts(Value const&) const = 0;
 
-    virtual String format_range(Value const&, Value const&) const = 0;
+    virtual Utf16String format_range(Value const&, Value const&) const = 0;
     virtual Vector<Partition> format_range_to_parts(Value const&, Value const&) const = 0;
 
     virtual void create_plural_rules(PluralForm) = 0;