diff --git a/lib/internal/Magento/Framework/Test/Unit/TranslateTest.php b/lib/internal/Magento/Framework/Test/Unit/TranslateTest.php
new file mode 100644
index 0000000000000..885be8557fa10
--- /dev/null
+++ b/lib/internal/Magento/Framework/Test/Unit/TranslateTest.php
@@ -0,0 +1,373 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\Framework\Test\Unit;
+
+use Magento\Framework\Serialize\SerializerInterface;
+use Magento\Framework\Translate;
+
+/**
+ * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
+ */
+class TranslateTest extends \PHPUnit\Framework\TestCase
+{
+    /** @var Translate */
+    protected $translate;
+
+    /** @var \Magento\Framework\View\DesignInterface|\PHPUnit_Framework_MockObject_MockObject */
+    protected $viewDesign;
+
+    /** @var \Magento\Framework\Cache\FrontendInterface|\PHPUnit_Framework_MockObject_MockObject */
+    protected $cache;
+
+    /** @var \Magento\Framework\View\FileSystem|\PHPUnit_Framework_MockObject_MockObject */
+    protected $viewFileSystem;
+
+    /** @var \Magento\Framework\Module\ModuleList|\PHPUnit_Framework_MockObject_MockObject */
+    protected $moduleList;
+
+    /** @var \Magento\Framework\Module\Dir\Reader|\PHPUnit_Framework_MockObject_MockObject */
+    protected $modulesReader;
+
+    /** @var \Magento\Framework\App\ScopeResolverInterface|\PHPUnit_Framework_MockObject_MockObject */
+    protected $scopeResolver;
+
+    /** @var \Magento\Framework\Translate\ResourceInterface|\PHPUnit_Framework_MockObject_MockObject */
+    protected $resource;
+
+    /** @var \Magento\Framework\Locale\ResolverInterface|\PHPUnit_Framework_MockObject_MockObject */
+    protected $locale;
+
+    /** @var \Magento\Framework\App\State|\PHPUnit_Framework_MockObject_MockObject */
+    protected $appState;
+
+    /** @var \Magento\Framework\Filesystem|\PHPUnit_Framework_MockObject_MockObject */
+    protected $filesystem;
+
+    /** @var \Magento\Framework\App\RequestInterface|\PHPUnit_Framework_MockObject_MockObject */
+    protected $request;
+
+    /** @var \Magento\Framework\File\Csv|\PHPUnit_Framework_MockObject_MockObject */
+    protected $csvParser;
+
+    /** @var  \Magento\Framework\App\Language\Dictionary|\PHPUnit_Framework_MockObject_MockObject */
+    protected $packDictionary;
+
+    /** @var \Magento\Framework\Filesystem\Directory\ReadInterface|\PHPUnit_Framework_MockObject_MockObject */
+    protected $directory;
+
+    /** @var \Magento\Framework\Filesystem\DriverInterface|\PHPUnit_Framework_MockObject_MockObject */
+    protected $fileDriver;
+
+    protected function setUp(): void
+    {
+        $objectManager = new \Magento\Framework\TestFramework\Unit\Helper\ObjectManager($this);
+        $this->viewDesign = $this->createMock(\Magento\Framework\View\DesignInterface::class);
+        $this->cache = $this->createMock(\Magento\Framework\Cache\FrontendInterface::class);
+        $this->viewFileSystem = $this->createMock(\Magento\Framework\View\FileSystem::class);
+        $this->moduleList = $this->createMock(\Magento\Framework\Module\ModuleList::class);
+        $this->modulesReader = $this->createMock(\Magento\Framework\Module\Dir\Reader::class);
+        $this->scopeResolver = $this->createMock(\Magento\Framework\App\ScopeResolverInterface::class);
+        $this->resource = $this->createMock(\Magento\Framework\Translate\ResourceInterface::class);
+        $this->locale = $this->createMock(\Magento\Framework\Locale\ResolverInterface::class);
+        $this->appState = $this->createMock(\Magento\Framework\App\State::class);
+        $this->request = $this->getMockForAbstractClass(
+            \Magento\Framework\App\RequestInterface::class,
+            [],
+            '',
+            false,
+            false,
+            true,
+            ['getParam', 'getControllerModule']
+        );
+        $this->csvParser = $this->createMock(\Magento\Framework\File\Csv::class);
+        $this->packDictionary = $this->createMock(\Magento\Framework\App\Language\Dictionary::class);
+        $this->directory = $this->createMock(\Magento\Framework\Filesystem\Directory\ReadInterface::class);
+        $filesystem = $this->createMock(\Magento\Framework\Filesystem::class);
+        $filesystem->expects($this->once())->method('getDirectoryRead')->will($this->returnValue($this->directory));
+        $this->fileDriver = $this->createMock(\Magento\Framework\Filesystem\DriverInterface::class);
+
+        $this->translate = new Translate(
+            $this->viewDesign,
+            $this->cache,
+            $this->viewFileSystem,
+            $this->moduleList,
+            $this->modulesReader,
+            $this->scopeResolver,
+            $this->resource,
+            $this->locale,
+            $this->appState,
+            $filesystem,
+            $this->request,
+            $this->csvParser,
+            $this->packDictionary,
+            $this->fileDriver
+        );
+
+        $serializerMock = $this->createMock(SerializerInterface::class);
+        $serializerMock->method('serialize')
+            ->willReturnCallback(function ($data) {
+                return json_encode($data);
+            });
+        $serializerMock->method('unserialize')
+            ->willReturnCallback(function ($string) {
+                return json_decode($string, true);
+            });
+        $objectManager->setBackwardCompatibleProperty(
+            $this->translate,
+            'serializer',
+            $serializerMock
+        );
+    }
+
+    /**
+     * @param string $area
+     * @param bool $forceReload
+     * @param array $cachedData
+     * @dataProvider dataProviderLoadDataCachedTranslation
+     */
+    public function testLoadDataCachedTranslation($area, $forceReload, $cachedData): void
+    {
+        $this->expectsSetConfig('Magento/luma');
+
+        $this->cache->expects($this->once())
+            ->method('load')
+            ->will($this->returnValue(json_encode($cachedData)));
+
+        $this->appState->expects($this->exactly($area ? 0 : 1))
+            ->method('getAreaCode')
+            ->will($this->returnValue('frontend'));
+
+        $this->translate->loadData($area, $forceReload);
+        $this->assertEquals($cachedData, $this->translate->getData());
+    }
+
+    /**
+     * @return array
+     */
+    public function dataProviderLoadDataCachedTranslation(): array
+    {
+        $cachedData = ['cached 1' => 'translated 1', 'cached 2' => 'translated 2'];
+        return [
+            ['adminhtml', false, $cachedData],
+            ['frontend', false, $cachedData],
+            [null, false, $cachedData],
+        ];
+    }
+
+    /**
+     * @param string $area
+     * @param bool $forceReload
+     * @dataProvider dataProviderForTestLoadData
+     * @SuppressWarnings(PHPMD.NPathComplexity)
+     */
+    public function testLoadData($area, $forceReload): void
+    {
+        $this->expectsSetConfig('Magento/luma');
+
+        $this->appState->expects($this->exactly($area ? 0 : 1))
+            ->method('getAreaCode')
+            ->will($this->returnValue('frontend'));
+
+        $this->cache->expects($this->exactly($forceReload ? 0 : 1))
+            ->method('load')
+            ->will($this->returnValue(false));
+
+        $this->directory->expects($this->any())->method('isExist')->will($this->returnValue(true));
+
+        // _loadModuleTranslation()
+        $modules = ['some_module', 'other_module', 'another_module', 'current_module'];
+        $this->request->expects($this->any())
+            ->method('getControllerModule')
+            ->willReturn('current_module');
+        $this->moduleList->expects($this->once())->method('getNames')->will($this->returnValue($modules));
+        $moduleData = [
+            'module original' => 'module translated',
+            'module theme' => 'module-theme original translated',
+            'module pack' => 'module-pack original translated',
+            'module db' => 'module-db original translated',
+        ];
+        $this->modulesReader->expects($this->any())->method('getModuleDir')->will($this->returnValue('/app/module'));
+        $themeData = [
+            'theme original' => 'theme translated',
+            'module theme' => 'theme translated overwrite',
+            'module pack' => 'theme-pack translated overwrite',
+            'module db' => 'theme-db translated overwrite',
+        ];
+        $this->csvParser->expects($this->any())
+            ->method('getDataPairs')
+            ->will(
+                $this->returnValueMap(
+                    [
+                        ['/app/module/en_US.csv', 0, 1, $moduleData],
+                        ['/app/module/en_GB.csv', 0, 1, $moduleData],
+                        ['/theme.csv', 0, 1, $themeData],
+                    ]
+                )
+            );
+        $this->fileDriver->expects($this->any())
+            ->method('isExists')
+            ->will(
+                $this->returnValueMap(
+                    [
+                        ['/app/module/en_US.csv', true],
+                        ['/app/module/en_GB.csv', true],
+                        ['/theme.csv', true],
+                    ]
+                )
+            );
+
+        // _loadPackTranslation
+        $packData = [
+            'pack original' => 'pack translated',
+            'module pack' => 'pack translated overwrite',
+            'module db' => 'pack-db translated overwrite',
+        ];
+        $this->packDictionary->expects($this->once())->method('getDictionary')->will($this->returnValue($packData));
+
+        // _loadThemeTranslation()
+        $this->viewFileSystem->expects($this->any())
+            ->method('getLocaleFileName')
+            ->will($this->returnValue('/theme.csv'));
+
+        // _loadDbTranslation()
+        $dbData = [
+            'db original' => 'db translated',
+            'module db' => 'db translated overwrite',
+        ];
+        $this->resource->expects($this->any())->method('getTranslationArray')->will($this->returnValue($dbData));
+
+        $this->cache->expects($this->exactly($forceReload ? 0 : 1))->method('save');
+
+        $this->translate->loadData($area, $forceReload);
+
+        $expected = [
+            'module original' => 'module translated',
+            'module theme' => 'theme translated overwrite',
+            'module pack' => 'theme-pack translated overwrite',
+            'module db' => 'db translated overwrite',
+            'theme original' => 'theme translated',
+            'pack original' => 'pack translated',
+            'db original' => 'db translated',
+        ];
+        $this->assertEquals($expected, $this->translate->getData());
+    }
+
+    /**
+     * @return array
+     */
+    public function dataProviderForTestLoadData(): array
+    {
+        return [
+            ['adminhtml', true],
+            ['adminhtml', false],
+            ['frontend', true],
+            ['frontend', false],
+            [null, true],
+            [null, false]
+        ];
+    }
+
+    /**
+     * @param $data
+     * @param $result
+     * @dataProvider dataProviderForTestGetData
+     */
+    public function testGetData($data, $result): void
+    {
+        $this->cache->expects($this->once())
+            ->method('load')
+            ->will($this->returnValue(json_encode($data)));
+        $this->expectsSetConfig('themeId');
+        $this->translate->loadData('frontend');
+        $this->assertEquals($result, $this->translate->getData());
+    }
+
+    /**
+     * @return array
+     */
+    public function dataProviderForTestGetData(): array
+    {
+        $data = ['original 1' => 'translated 1', 'original 2' => 'translated 2'];
+        return [
+            [$data, $data],
+            [null, []]
+        ];
+    }
+
+    public function testGetLocale(): void
+    {
+        $this->locale->expects($this->once())->method('getLocale')->will($this->returnValue('en_US'));
+        $this->assertEquals('en_US', $this->translate->getLocale());
+
+        $this->locale->expects($this->never())->method('getLocale');
+        $this->assertEquals('en_US', $this->translate->getLocale());
+
+        $this->locale->expects($this->never())->method('getLocale');
+        $this->translate->setLocale('en_GB');
+        $this->assertEquals('en_GB', $this->translate->getLocale());
+    }
+
+    public function testSetLocale(): void
+    {
+        $this->translate->setLocale('en_GB');
+        $this->locale->expects($this->never())->method('getLocale');
+        $this->assertEquals('en_GB', $this->translate->getLocale());
+    }
+
+    public function testGetTheme(): void
+    {
+        $this->request->expects($this->at(0))->method('getParam')->with('theme')->will($this->returnValue(''));
+
+        $requestTheme = ['theme_title' => 'Theme Title'];
+        $this->request->expects($this->at(1))->method('getParam')->with('theme')
+            ->will($this->returnValue($requestTheme));
+
+        $this->assertEquals('theme', $this->translate->getTheme());
+        $this->assertEquals('themeTheme Title', $this->translate->getTheme());
+    }
+
+    public function testLoadDataNoTheme(): void
+    {
+        $forceReload = true;
+        $this->expectsSetConfig(null, null);
+        $this->moduleList->expects($this->once())->method('getNames')->will($this->returnValue([]));
+        $this->appState->expects($this->once())->method('getAreaCode')->will($this->returnValue('frontend'));
+        $this->packDictionary->expects($this->once())->method('getDictionary')->will($this->returnValue([]));
+        $this->resource->expects($this->any())->method('getTranslationArray')->will($this->returnValue([]));
+        $this->assertEquals($this->translate, $this->translate->loadData(null, $forceReload));
+    }
+
+    /**
+     * Declare calls expectation for setConfig() method
+     */
+    protected function expectsSetConfig($themeId, $localeCode = 'en_US'): void
+    {
+        $this->locale->expects($this->any())->method('getLocale')->will($this->returnValue($localeCode));
+        $scope = new \Magento\Framework\DataObject(['code' => 'frontendCode', 'id' => 1]);
+        $scopeAdmin = new \Magento\Framework\DataObject(['code' => 'adminCode', 'id' => 0]);
+        $this->scopeResolver->expects($this->any())
+            ->method('getScope')
+            ->will(
+                $this->returnValueMap(
+                    [
+                        [null, $scope],
+                        ['admin', $scopeAdmin],
+                    ]
+                )
+            );
+        $designTheme = $this->getMockBuilder(\Magento\Theme\Model\Theme::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        $designTheme->expects($this->once())
+            ->method('getThemePath')
+            ->willReturn($themeId);
+
+        $this->viewDesign->expects($this->any())->method('getDesignTheme')->will($this->returnValue($designTheme));
+    }
+}
diff --git a/lib/internal/Magento/Framework/Translate.php b/lib/internal/Magento/Framework/Translate.php
index ff1bf99162a8a..ce73338eee292 100644
--- a/lib/internal/Magento/Framework/Translate.php
+++ b/lib/internal/Magento/Framework/Translate.php
@@ -398,11 +398,11 @@ protected function _getModuleTranslationFile($moduleName, $locale)
     /**
      * Get theme translation locale file name
      *
-     * @param string $locale
+     * @param string|null $locale
      * @param array $config
      * @return string|null
      */
-    private function getThemeTranslationFileName(string $locale, array $config): ?string
+    private function getThemeTranslationFileName(?string $locale, array $config): ?string
     {
         $fileName = $this->_viewFileSystem->getLocaleFileName(
             'i18n' . '/' . $locale . '.csv',