diff --git a/.gitignore b/.gitignore
index 7d5e145..fff9157 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,6 @@
 .idea/
-composer.lock
 .phpcs.cache
+.phpunit.cache
+composer.lock
+tests/coverage
 vendor/
\ No newline at end of file
diff --git a/README.md b/README.md
index 68b949f..0e1e6e0 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,4 @@
 # Composer i18n Scripts
 
 Simplify the internationalization of your WordPress plugin or theme using WP-CLI — powered by Composer.
+
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index 0ad9c3e..9d8a25f 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -1,4 +1,4 @@
 parameters:
-    level: 8
+    level: 10
     paths:
         - src/
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 5e76c56..ce0df9c 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,10 +1,25 @@
-<!-- phpunit.xml.dist -->
-<phpunit bootstrap="vendor/autoload.php"
-         colors="true"
-         stopOnFailure="false">
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         cacheDirectory=".phpunit.cache"
+         executionOrder="depends,defects"
+         requireCoverageMetadata="true"
+         beStrictAboutCoverageMetadata="true"
+         beStrictAboutOutputDuringTests="true"
+         displayDetailsOnPhpunitDeprecations="true"
+         failOnPhpunitDeprecation="true"
+         failOnRisky="true"
+         failOnWarning="true">
     <testsuites>
-        <testsuite name="Unit">
+        <testsuite name="default">
             <directory>tests</directory>
         </testsuite>
     </testsuites>
-</phpunit>
\ No newline at end of file
+
+    <source ignoreIndirectDeprecations="true" restrictNotices="true" restrictWarnings="true">
+        <include>
+            <directory>src</directory>
+        </include>
+    </source>
+</phpunit>
diff --git a/src/Commands/CreatePoCommand.php b/src/Commands/CreatePoCommand.php
index 791c9a7..aac1f7c 100644
--- a/src/Commands/CreatePoCommand.php
+++ b/src/Commands/CreatePoCommand.php
@@ -18,6 +18,8 @@
  */
 class CreatePoCommand extends CustomCommand {
 
+    const DEFAULT_LANGUAGE = 'invalid_LOCALE';
+
 	/**
 	 * Configures the command.
 	 */
@@ -54,6 +56,9 @@ protected function configure(): void {
 	 */
 	protected function command( I18nConfig $config, InputInterface $input ): string {
 		$locale = $input->getArgument( 'locale' );
+        if ( empty( $locale ) || ! is_string( $locale ) ) {
+            $locale = self::DEFAULT_LANGUAGE;
+        }
 
 		return sprintf(
 			'if [ ! -e %1$s ]; then cp %2$s %1$s; fi',
diff --git a/src/Commands/CustomCommand.php b/src/Commands/CustomCommand.php
index 714be71..6b255e8 100644
--- a/src/Commands/CustomCommand.php
+++ b/src/Commands/CustomCommand.php
@@ -69,7 +69,10 @@ protected function execute( InputInterface $input, OutputInterface $output ): in
 
 		$output->writeln( '<info>Running: ' . $command . '</info>' );
 
-		$this->runner->exec( $command, $output_lines, $exit_code );
+        $output_lines = [];
+        $exit_code = 0;
+
+        $this->runner->exec( $command, $output_lines, $exit_code );
 
 		foreach ( $output_lines as $line ) {
 			$output->writeln( $line );
diff --git a/src/ShellRunner.php b/src/ShellRunner.php
index b26470d..4869831 100644
--- a/src/ShellRunner.php
+++ b/src/ShellRunner.php
@@ -18,11 +18,11 @@ class ShellRunner {
 	 * Executes a shell command.
 	 *
 	 * @param string    $command The command to execute.
-	 * @param ?string[] $output The output in an array.
-	 * @param ?int      $result_code The result code of the command execution.
+	 * @param string[] $output The output in an array.
+	 * @param int      $result_code The result code of the command execution.
 	 * @return void
 	 */
-	public function exec( string $command, ?array &$output, ?int &$result_code ): void {
+	public function exec( string $command, array &$output, int &$result_code ): void {
         // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.system_calls_exec
 		exec( $command, $output, $result_code );
 	}
diff --git a/tests/Commands/CreatePoCommandTest.php b/tests/Commands/CreatePoCommandTest.php
new file mode 100644
index 0000000..731a724
--- /dev/null
+++ b/tests/Commands/CreatePoCommandTest.php
@@ -0,0 +1,44 @@
+<?php
+/**
+ * Test of MakePotCommand
+ */
+
+declare(strict_types=1);
+
+namespace Commands;
+
+use lloc\ComposerI18nScripts\Commands\CreatePoCommand;
+
+use lloc\ComposerI18nScripts\ShellRunner;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Console\Application;
+
+#[CoversClass(CreatePoCommand::class)]
+class CreatePoCommandTest extends TestCase {
+
+    protected Application $application;
+
+    protected function setUp(): void {
+        $runner = $this->createMock(ShellRunner::class);
+        $runner->method('exec')
+            ->willReturnCallback(function ($cmd, &$output, &$exitCode) {
+                $output = ['Mocked .po creation'];
+                $exitCode = 0;
+            });
+
+        $command = new CreatePoCommand($runner);
+
+        $this->application = new Application();
+        $this->application->setAutoExit(false); // Important for tests
+        $this->application->add($command);
+    }
+
+	public function testCommandIsRegisteredAndConfigured(): void {
+		$command = $this->application->find( 'i18n:create-po' );
+
+		$this->assertSame( 'i18n:create-po', $command->getName() );
+		$this->assertNotEmpty( $command->getDescription() );
+		$this->assertNotEmpty( $command->getHelp() );
+	}
+}
diff --git a/tests/Commands/MakeJsonCommandTest.php b/tests/Commands/MakeJsonCommandTest.php
new file mode 100644
index 0000000..d3e9192
--- /dev/null
+++ b/tests/Commands/MakeJsonCommandTest.php
@@ -0,0 +1,44 @@
+<?php
+/**
+ * Test of MakeJsonCommand
+ */
+
+declare(strict_types=1);
+
+namespace lloc\ComposerI18nScriptsTests\Commands;
+
+use lloc\ComposerI18nScripts\Commands\MakeJsonCommand;
+use lloc\ComposerI18nScripts\ShellRunner;
+use PHPUnit\Framework\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+use Symfony\Component\Console\Application;
+
+#[CoversClass(MakeJsonCommand::class)]
+class MakeJsonCommandTest extends TestCase {
+
+    protected Application $application;
+
+    protected function setUp(): void {
+        $runner = $this->createMock(ShellRunner::class);
+        $runner->method('exec')
+            ->willReturnCallback(function ($cmd, &$output, &$exitCode) {
+                $output = ['Mocked .json generation'];
+                $exitCode = 0;
+            });
+
+        $command = new MakeJsonCommand($runner);
+
+        $this->application = new Application();
+        $this->application->setAutoExit(false); // Important for tests
+        $this->application->add($command);
+    }
+
+
+    public function testCommandIsRegisteredAndConfigured(): void {
+        $command = $this->application->find( 'i18n:make-json' );
+
+        $this->assertSame( 'i18n:make-json', $command->getName() );
+        $this->assertNotEmpty( $command->getDescription() );
+        $this->assertNotEmpty( $command->getHelp() );
+    }
+}
diff --git a/tests/Commands/MakeMoCommandTest.php b/tests/Commands/MakeMoCommandTest.php
new file mode 100644
index 0000000..d6e2594
--- /dev/null
+++ b/tests/Commands/MakeMoCommandTest.php
@@ -0,0 +1,43 @@
+<?php
+/**
+ * Test of MakeMoCommand
+ */
+
+declare(strict_types=1);
+
+namespace Commands;
+
+use lloc\ComposerI18nScripts\Commands\MakeMoCommand;
+use lloc\ComposerI18nScripts\ShellRunner;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Console\Application;
+
+#[CoversClass(MakeMoCommand::class)]
+class MakeMoCommandTest extends TestCase {
+
+    protected Application $application;
+
+    protected function setUp(): void {
+        $runner = $this->createMock(ShellRunner::class);
+        $runner->method('exec')
+            ->willReturnCallback(function ($cmd, &$output, &$exitCode) {
+                $output = ['Mocked .mo generation'];
+                $exitCode = 0;
+            });
+
+        $command = new MakeMoCommand($runner);
+
+        $this->application = new Application();
+        $this->application->setAutoExit(false); // Important for tests
+        $this->application->add($command);
+    }
+
+    public function testCommandIsRegisteredAndConfigured(): void {
+        $command = $this->application->find( 'i18n:make-mo' );
+
+        $this->assertSame( 'i18n:make-mo', $command->getName() );
+        $this->assertNotEmpty( $command->getDescription() );
+        $this->assertNotEmpty( $command->getHelp() );
+    }
+}
diff --git a/tests/Commands/MakePhpCommandTest.php b/tests/Commands/MakePhpCommandTest.php
new file mode 100644
index 0000000..0bc7c9f
--- /dev/null
+++ b/tests/Commands/MakePhpCommandTest.php
@@ -0,0 +1,44 @@
+<?php
+/**
+ * Test of MakePotCommand
+ */
+
+declare(strict_types=1);
+
+namespace Commands;
+
+use lloc\ComposerI18nScripts\Commands\MakePhpCommand;
+
+use lloc\ComposerI18nScripts\ShellRunner;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Console\Application;
+
+#[CoversClass(MakePhpCommand::class)]
+class MakePhpCommandTest extends TestCase {
+
+    protected Application $application;
+
+    protected function setUp(): void {
+        $runner = $this->createMock(ShellRunner::class);
+        $runner->method('exec')
+            ->willReturnCallback(function ($cmd, &$output, &$exitCode) {
+                $output = ['Mocked .php generation'];
+                $exitCode = 0;
+            });
+
+        $command = new MakePhpCommand($runner);
+
+        $this->application = new Application();
+        $this->application->setAutoExit(false); // Important for tests
+        $this->application->add($command);
+    }
+
+	public function testCommandIsRegisteredAndConfigured(): void {
+		$command = $this->application->find( 'i18n:make-php' );
+
+		$this->assertSame( 'i18n:make-php', $command->getName() );
+		$this->assertNotEmpty( $command->getDescription() );
+		$this->assertNotEmpty( $command->getHelp() );
+	}
+}
diff --git a/tests/Commands/MakePotCommandTest.php b/tests/Commands/MakePotCommandTest.php
index 9927185..2a10780 100644
--- a/tests/Commands/MakePotCommandTest.php
+++ b/tests/Commands/MakePotCommandTest.php
@@ -10,35 +10,35 @@
 use lloc\ComposerI18nScripts\Commands\MakePotCommand;
 
 use lloc\ComposerI18nScripts\ShellRunner;
+use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Console\Application;
-use Symfony\Component\Console\Tester\CommandTester;
 
+#[CoversClass(MakePotCommand::class)]
 class MakePotCommandTest extends TestCase {
 
-	public function testCommandIsRegisteredAndConfigured(): void {
-        $runner = new ShellRunner();
-		$application = new Application();
-		$application->add( new MakePotCommand($runner) );
+    protected Application $application;
 
-		$command = $application->find( 'i18n:make-pot' );
-		$this->assertSame( 'i18n:make-pot', $command->getName() );
-		$this->assertNotEmpty( $command->getDescription() );
-		$this->assertNotEmpty( $command->getHelp() );
-	}
+    protected function setUp(): void {
+        $runner = $this->createMock(ShellRunner::class);
+        $runner->method('exec')
+            ->willReturnCallback(function ($cmd, &$output, &$exitCode) {
+                $output = ['Mocked .pot generation'];
+                $exitCode = 0;
+            });
 
-	public function testCommandExecutionReturnsSuccess(): void {
-        $runner = new ShellRunner();
-		$application = new Application();
-		$application->add( new MakePotCommand($runner) );
+        $command = new MakePotCommand($runner);
 
-		$command = $application->find( 'i18n:make-pot' );
-		$tester  = new CommandTester( $command );
+        $this->application = new Application();
+        $this->application->setAutoExit(false); // Important for tests
+        $this->application->add($command);
+    }
 
-		// You may need to mock or override shell exec inside the command to make this testable
-		$tester->execute( array() );
+	public function testCommandIsRegisteredAndConfigured(): void {
+		$command = $this->application->find( 'i18n:make-pot' );
 
-		// Just test if it runs (for now)
-		$this->assertSame( 0, $tester->getStatusCode() );
+		$this->assertSame( 'i18n:make-pot', $command->getName() );
+		$this->assertNotEmpty( $command->getDescription() );
+		$this->assertNotEmpty( $command->getHelp() );
 	}
 }
diff --git a/tests/Commands/UpdatePoCommandTest.php b/tests/Commands/UpdatePoCommandTest.php
new file mode 100644
index 0000000..2d4c3f7
--- /dev/null
+++ b/tests/Commands/UpdatePoCommandTest.php
@@ -0,0 +1,43 @@
+<?php
+/**
+ * Test of MakePotCommand
+ */
+
+declare(strict_types=1);
+
+namespace Commands;
+
+use lloc\ComposerI18nScripts\Commands\UpdatePoCommand;
+use lloc\ComposerI18nScripts\ShellRunner;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Console\Application;
+
+#[CoversClass(UpdatePoCommand::class)]
+class UpdatePoCommandTest extends TestCase {
+
+    protected Application $application;
+
+    protected function setUp(): void {
+        $runner = $this->createMock(ShellRunner::class);
+        $runner->method('exec')
+            ->willReturnCallback(function ($cmd, &$output, &$exitCode) {
+                $output = ['Mocked .po update'];
+                $exitCode = 0;
+            });
+
+        $command = new UpdatePoCommand($runner);
+
+        $this->application = new Application();
+        $this->application->setAutoExit(false); // Important for tests
+        $this->application->add($command);
+    }
+
+	public function testCommandIsRegisteredAndConfigured(): void {
+		$command = $this->application->find( 'i18n:update-po' );
+
+		$this->assertSame( 'i18n:update-po', $command->getName() );
+		$this->assertNotEmpty( $command->getDescription() );
+		$this->assertNotEmpty( $command->getHelp() );
+	}
+}