Add locale in DatePickerThemeData (#148292)

*This PR changes the date picker were add locale in DatePickerThemeData*

*List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.*
Fixes #148202 

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

packages/flutter/lib/src/material/date_picker.dart

@@ -241,6 +241,15 @@ Future<DateTime?> showDatePicker({
       locale: locale,
       child: dialog,
     );
+  } else {
+    final DatePickerThemeData datePickerTheme = DatePickerTheme.of(context);
+    if (datePickerTheme.locale != null) {
+      dialog = Localizations.override(
+        context: context,
+        locale: datePickerTheme.locale,
+        child: dialog,
+      );
+    }
   }
 
   return showDialog<DateTime>(

packages/flutter/lib/src/material/date_picker_theme.dart

@@ -75,6 +75,7 @@ class DatePickerThemeData with Diagnosticable {
     this.inputDecorationTheme,
     this.cancelButtonStyle,
     this.confirmButtonStyle,
+    this.locale,
   });
 
   /// Overrides the default value of [Dialog.backgroundColor].
@@ -347,6 +348,10 @@ class DatePickerThemeData with Diagnosticable {
   /// Overrides the default style of the confirm (OK) button of a [DatePickerDialog].
   final ButtonStyle? confirmButtonStyle;
 
+  /// An optional [locale] argument can be used to set the locale for the date
+  /// picker. It defaults to the ambient locale provided by [Localizations].
+  final Locale? locale;
+
   /// Creates a copy of this object with the given fields replaced with the
   /// new values.
   DatePickerThemeData copyWith({
@@ -387,6 +392,7 @@ class DatePickerThemeData with Diagnosticable {
     InputDecorationTheme? inputDecorationTheme,
     ButtonStyle? cancelButtonStyle,
     ButtonStyle? confirmButtonStyle,
+    Locale? locale,
   }) {
     return DatePickerThemeData(
       backgroundColor: backgroundColor ?? this.backgroundColor,
@@ -426,6 +432,7 @@ class DatePickerThemeData with Diagnosticable {
       inputDecorationTheme: inputDecorationTheme ?? this.inputDecorationTheme,
       cancelButtonStyle: cancelButtonStyle ?? this.cancelButtonStyle,
       confirmButtonStyle: confirmButtonStyle ?? this.confirmButtonStyle,
+      locale: locale ?? this.locale,
     );
   }
 
@@ -472,6 +479,7 @@ class DatePickerThemeData with Diagnosticable {
       inputDecorationTheme: t < 0.5 ? a?.inputDecorationTheme : b?.inputDecorationTheme,
       cancelButtonStyle: ButtonStyle.lerp(a?.cancelButtonStyle, b?.cancelButtonStyle, t),
       confirmButtonStyle: ButtonStyle.lerp(a?.confirmButtonStyle, b?.confirmButtonStyle, t),
+      locale: t < 0.5 ? a?.locale : b?.locale,
     );
   }
 
@@ -524,6 +532,7 @@ class DatePickerThemeData with Diagnosticable {
     inputDecorationTheme,
     cancelButtonStyle,
     confirmButtonStyle,
+    locale,
   ]);
 
   @override
@@ -568,7 +577,8 @@ class DatePickerThemeData with Diagnosticable {
       && other.dividerColor == dividerColor
       && other.inputDecorationTheme == inputDecorationTheme
       && other.cancelButtonStyle == cancelButtonStyle
-      && other.confirmButtonStyle == confirmButtonStyle;
+      && other.confirmButtonStyle == confirmButtonStyle
+      && other.locale == locale;
   }
 
   @override
@@ -611,6 +621,7 @@ class DatePickerThemeData with Diagnosticable {
     properties.add(DiagnosticsProperty<InputDecorationTheme>('inputDecorationTheme', inputDecorationTheme, defaultValue: null));
     properties.add(DiagnosticsProperty<ButtonStyle>('cancelButtonStyle', cancelButtonStyle, defaultValue: null));
     properties.add(DiagnosticsProperty<ButtonStyle>('confirmButtonStyle', confirmButtonStyle, defaultValue: null));
+    properties.add(DiagnosticsProperty<Locale>('locale', locale, defaultValue: null));
   }
 }
 

packages/flutter/test/material/date_picker_theme_test.dart

@@ -50,6 +50,7 @@ void main() {
     ),
     cancelButtonStyle: ButtonStyle(foregroundColor: MaterialStatePropertyAll<Color>(Color(0xffffff6f))),
     confirmButtonStyle: ButtonStyle(foregroundColor: MaterialStatePropertyAll<Color>(Color(0xffffff7f))),
+    locale: Locale('en'),
   );
 
   Material findDialogMaterial(WidgetTester tester) {
@@ -145,6 +146,7 @@ void main() {
     expect(theme.inputDecorationTheme, null);
     expect(theme.cancelButtonStyle, null);
     expect(theme.confirmButtonStyle, null);
+    expect(theme.locale, null);
   });
 
   testWidgets('DatePickerTheme.defaults M3 defaults', (WidgetTester tester) async {
@@ -223,6 +225,7 @@ void main() {
     expect(m3.inputDecorationTheme, null);
     expect(m3.cancelButtonStyle.toString(), equalsIgnoringHashCodes(TextButton.styleFrom().toString()));
     expect(m3.confirmButtonStyle.toString(), equalsIgnoringHashCodes(TextButton.styleFrom().toString()));
+    expect(m3.locale, null);
   });
 
   testWidgets('DatePickerTheme.defaults M2 defaults', (WidgetTester tester) async {
@@ -293,6 +296,7 @@ void main() {
     expect(m2.inputDecorationTheme, null);
     expect(m2.cancelButtonStyle.toString(), equalsIgnoringHashCodes(TextButton.styleFrom().toString()));
     expect(m2.confirmButtonStyle.toString(), equalsIgnoringHashCodes(TextButton.styleFrom().toString()));
+    expect(m2.locale, null);
   });
 
   testWidgets('Default DatePickerThemeData debugFillProperties', (WidgetTester tester) async {
@@ -354,7 +358,8 @@ void main() {
       'dividerColor: Color(0xffffff4f)',
       'inputDecorationTheme: InputDecorationTheme#00000(fillColor: Color(0xffffff5f), border: UnderlineInputBorder())',
       'cancelButtonStyle: ButtonStyle#00000(foregroundColor: WidgetStatePropertyAll(Color(0xffffff6f)))',
-      'confirmButtonStyle: ButtonStyle#00000(foregroundColor: WidgetStatePropertyAll(Color(0xffffff7f)))'
+      'confirmButtonStyle: ButtonStyle#00000(foregroundColor: WidgetStatePropertyAll(Color(0xffffff7f)))',
+      'locale: en',
     ]));
   });