diff --git a/CHANGELOG.md b/CHANGELOG.md
index e960d20..230827e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 - Enh #99: In `SimpleMessageFormatter` add support of messages where used parameters with plural modifier that contain 
   non-supported keys (@vjik)
+- Bug #110: Changed default value for locale to BCP-47 compatible (@darkdef)
 
 ## 2.2.0 November 28, 2022
 
diff --git a/config/params.php b/config/params.php
index 83a3e71..73fe652 100644
--- a/config/params.php
+++ b/config/params.php
@@ -4,7 +4,7 @@
 
 return [
     'yiisoft/translator' => [
-        'locale' => 'en_US',
+        'locale' => 'en-US',
         'fallbackLocale' => null,
         'defaultCategory' => 'app',
     ],
diff --git a/src/SimpleMessageFormatter.php b/src/SimpleMessageFormatter.php
index 411c436..36d1d93 100644
--- a/src/SimpleMessageFormatter.php
+++ b/src/SimpleMessageFormatter.php
@@ -16,7 +16,7 @@ class SimpleMessageFormatter implements MessageFormatterInterface
     private const PLURAL_OTHER = 'other';
     private const PLURAL_KEYS = [self::PLURAL_ONE, self::PLURAL_OTHER];
 
-    public function format(string $message, array $parameters, string $locale = 'en_US'): string
+    public function format(string $message, array $parameters, string $locale = 'en-US'): string
     {
         preg_match_all('/{((?>[^{}]+)|(?R))*}/', $message, $matches);
         $replacements = [];
diff --git a/src/Translator.php b/src/Translator.php
index f75a073..a0451e8 100644
--- a/src/Translator.php
+++ b/src/Translator.php
@@ -36,7 +36,7 @@ final class Translator implements TranslatorInterface
      * @param EventDispatcherInterface|null $eventDispatcher Event dispatcher for translation events. Null for none.
      */
     public function __construct(
-        private string $locale = 'en_US',
+        private string $locale = 'en-US',
         private ?string $fallbackLocale = null,
         private string $defaultCategory = 'app',
         private ?EventDispatcherInterface $eventDispatcher = null,
diff --git a/tests/CategoryTest.php b/tests/CategoryTest.php
index eacd35c..868d64e 100644
--- a/tests/CategoryTest.php
+++ b/tests/CategoryTest.php
@@ -93,7 +93,7 @@ public function testWithoutFormatter(
         $categorySource = new CategorySource('test', $this->createMessageReader());
         $this->assertSame(
             $expectedMessage,
-            $categorySource->format($message, $parameters, 'en_US', $defaultMessageFormatter)
+            $categorySource->format($message, $parameters, 'en-US', $defaultMessageFormatter)
         );
     }
 
diff --git a/tests/ConfigTest.php b/tests/ConfigTest.php
index 086ecc6..c18bafd 100644
--- a/tests/ConfigTest.php
+++ b/tests/ConfigTest.php
@@ -37,11 +37,11 @@ public function testReset(): void
         $container = $this->createContainer();
 
         $translator = $container->get(TranslatorInterface::class);
-        $translator->setLocale('ru_RU');
+        $translator->setLocale('ru-RU');
 
         $container->get(StateResetter::class)->reset();
 
-        $this->assertSame('en_US', $translator->getLocale());
+        $this->assertSame('en-US', $translator->getLocale());
     }
 
     private function createContainer(?array $params = null, bool $withCategorySources = false): Container
diff --git a/tests/IdMessageReaderTest.php b/tests/IdMessageReaderTest.php
index 0bf32ce..3370c76 100644
--- a/tests/IdMessageReaderTest.php
+++ b/tests/IdMessageReaderTest.php
@@ -14,7 +14,7 @@ public function testGetMessage(): void
     {
         $reader = new IdMessageReader();
 
-        $this->assertSame('test', $reader->getMessage('test', 'my-module', 'en_US'));
+        $this->assertSame('test', $reader->getMessage('test', 'my-module', 'en-US'));
     }
 
     public function testGetMessages(): void
@@ -23,6 +23,6 @@ public function testGetMessages(): void
 
         $this->expectException(RuntimeException::class);
         $this->expectExceptionMessage('IdMessageReader doesn\'t support getting all messages at once');
-        $reader->getMessages('my-module', 'en_US');
+        $reader->getMessages('my-module', 'en-US');
     }
 }
diff --git a/tests/TranslatorTest.php b/tests/TranslatorTest.php
index 2409a9f..1b0c949 100644
--- a/tests/TranslatorTest.php
+++ b/tests/TranslatorTest.php
@@ -20,7 +20,18 @@ final class TranslatorTest extends TestCase
     public function testDefaultLocale(): void
     {
         $translator = new Translator();
-        $this->assertSame('en_US', $translator->getLocale());
+        $this->assertSame('en-US', $translator->getLocale());
+        $this->assertSame('test', $translator->translate('test'));
+    }
+
+    public function testDefaultLocaleWithCategorySource(): void
+    {
+        $translator = new Translator();
+        $this->assertSame('en-US', $translator->getLocale());
+
+        $categorySource = $this->createCategory('app', []);
+        $translator->addCategorySources($categorySource);
+        $this->assertSame('test', $translator->translate('test'));
     }
 
     private function getMessages(): array