diff --git a/framework/helpers/BaseFileHelper.php b/framework/helpers/BaseFileHelper.php
index 3e2d56a66c..e8f3d4beca 100644
--- a/framework/helpers/BaseFileHelper.php
+++ b/framework/helpers/BaseFileHelper.php
@@ -42,8 +42,8 @@ class BaseFileHelper
 	 * The searching is based on the specified language code. In particular,
 	 * a file with the same name will be looked for under the subdirectory
 	 * whose name is the same as the language code. For example, given the file "path/to/view.php"
-	 * and language code "zh_CN", the localized file will be looked for as
-	 * "path/to/zh_CN/view.php". If the file is not found, it will try a fallback with just a language code that is
+	 * and language code "zh-CN", the localized file will be looked for as
+	 * "path/to/zh-CN/view.php". If the file is not found, it will try a fallback with just a language code that is
 	 * "zh" i.e. "path/to/zh/view.php". If it is not found as well the original file will be returned.
 	 *
 	 * If the target and the source language codes are the same,
diff --git a/framework/i18n/DbMessageSource.php b/framework/i18n/DbMessageSource.php
index 69a5380af3..31fca124ea 100644
--- a/framework/i18n/DbMessageSource.php
+++ b/framework/i18n/DbMessageSource.php
@@ -162,7 +162,7 @@ class DbMessageSource extends MessageSource
 				->andWhere('t2.id NOT IN (SELECT id FROM '.$this->messageTable.' WHERE language = :language)')
 				->params([':category' => $category, ':language' => $language, ':fallbackLanguage' => $fallbackLanguage]);
 
-			$mainQuery->union($fallbackQuery);
+			$mainQuery->union($fallbackQuery, true);
 		}
 
 		$messages = $mainQuery->createCommand($this->db)->queryAll();
diff --git a/framework/i18n/GettextMessageSource.php b/framework/i18n/GettextMessageSource.php
index d7bd4900a4..c5b1f6f7e1 100644
--- a/framework/i18n/GettextMessageSource.php
+++ b/framework/i18n/GettextMessageSource.php
@@ -73,8 +73,8 @@ class GettextMessageSource extends MessageSource
 			} else if (empty($messages)) {
 				return $fallbackMessages;
 			} else if (!empty($fallbackMessages)) {
-				foreach ($messages as $key => $value) {
-					if (empty($value) && !empty($fallbackMessages[$key])) {
+				foreach ($fallbackMessages as $key => $value) {
+					if (!empty($value) && empty($messages[$key])) {
 						$messages[$key] = $fallbackMessages[$key];
 					}
 				}
diff --git a/framework/i18n/PhpMessageSource.php b/framework/i18n/PhpMessageSource.php
index 453ef79f85..2e611f50e0 100644
--- a/framework/i18n/PhpMessageSource.php
+++ b/framework/i18n/PhpMessageSource.php
@@ -76,8 +76,8 @@ class PhpMessageSource extends MessageSource
 			} else if (empty($messages)) {
 				return $fallbackMessages;
 			} else if (!empty($fallbackMessages)) {
-				foreach ($messages as $key => $value) {
-					if (empty($value) && !empty($fallbackMessages[$key])) {
+				foreach ($fallbackMessages as $key => $value) {
+					if (!empty($value) && empty($messages[$key])) {
 						$messages[$key] = $fallbackMessages[$key];
 					}
 				}
diff --git a/tests/unit/data/i18n/messages/de/test.php b/tests/unit/data/i18n/messages/de/test.php
new file mode 100644
index 0000000000..f2d6418c22
--- /dev/null
+++ b/tests/unit/data/i18n/messages/de/test.php
@@ -0,0 +1,7 @@
+<?php
+/**
+ *
+ */
+return [
+	'Hello world!' => 'Hallo Welt!',
+];
\ No newline at end of file
diff --git a/tests/unit/data/i18n/messages/ru/test.php b/tests/unit/data/i18n/messages/ru/test.php
new file mode 100644
index 0000000000..ca6ad202ec
--- /dev/null
+++ b/tests/unit/data/i18n/messages/ru/test.php
@@ -0,0 +1,7 @@
+<?php
+/**
+ *
+ */
+return [
+	'The dog runs fast.' => 'Собака бегает быстро.',
+];
\ No newline at end of file
diff --git a/tests/unit/framework/i18n/I18NTest.php b/tests/unit/framework/i18n/I18NTest.php
index aa2356be53..5d6a78b326 100644
--- a/tests/unit/framework/i18n/I18NTest.php
+++ b/tests/unit/framework/i18n/I18NTest.php
@@ -40,8 +40,18 @@ class I18NTest extends TestCase
 	public function testTranslate()
 	{
 		$msg = 'The dog runs fast.';
-		$this->assertEquals('The dog runs fast.', $this->i18n->translate('test', $msg, [], 'en-US'));
+
+		// source = target. Should be returned as is.
+		$this->assertEquals('The dog runs fast.', $this->i18n->translate('test', $msg, [], 'en'));
+
+		// exact match
 		$this->assertEquals('Der Hund rennt schnell.', $this->i18n->translate('test', $msg, [], 'de-DE'));
+
+		// fallback to just language code with absent exact match
+		$this->assertEquals('Собака бегает быстро.', $this->i18n->translate('test', $msg, [], 'ru-RU'));
+
+		// fallback to just langauge code with present exact match
+		$this->assertEquals('Hallo Welt!', $this->i18n->translate('test', 'Hello world!', [], 'de-DE'));
 	}
 
 	public function testTranslateParams()