diff --git src/wp-includes/l10n.php src/wp-includes/l10n.php
index 4bfb989..ad986e4 100644
--- src/wp-includes/l10n.php
+++ src/wp-includes/l10n.php
@@ -780,6 +780,50 @@ function load_child_theme_textdomain( $domain, $path = false ) {
 }
 
 /**
+ * Just in time loading of plugin and theme textdomains.
+ *
+ * When a textdomain is encountered for the first time, we try to load the translation file
+ * from wp-content/languages, removing the need to call @see load_plugin_texdomain() or
+ * @see load_theme_texdomain(). Holds a cached list of available .mo files to improve performance.
+ *
+ * @since 4.6.0
+ *
+ * @see get_translations_for_domain()
+ *
+ * @param string $domain Text domain. Unique identifier for retrieving translated strings.
+ * @return bool True when the textdomain is successfully loaded, false otherwise.
+ */
+function load_textdomain_just_in_time( $domain ) {
+	static $cached_mofiles = null;
+
+	if ( null === $cached_mofiles ) {
+		$locations = array(
+			WP_LANG_DIR . '/plugins',
+			WP_LANG_DIR . '/themes',
+		);
+
+		foreach ( $locations as $location ) {
+			foreach ( get_available_languages( $location ) as $file ) {
+				$cached_mofiles[] = "{$location}/{$file}.mo";
+			}
+		}
+	}
+
+	$locale = get_locale();
+	$mofile = "{$domain}-{$locale}.mo";
+
+	if ( in_array( WP_LANG_DIR . '/plugins/' . $mofile, $cached_mofiles ) ) {
+		return load_textdomain( $domain, WP_LANG_DIR . '/plugins/' . $mofile );
+	}
+
+	if ( in_array( WP_LANG_DIR . '/themes/' . $mofile, $cached_mofiles ) ) {
+		return load_textdomain( $domain, WP_LANG_DIR . '/themes/' . $mofile );
+	}
+
+	return false;
+}
+
+/**
  * Return the Translations instance for a text domain.
  *
  * If there isn't one, returns empty Translations instance.
@@ -793,7 +837,7 @@ function load_child_theme_textdomain( $domain, $path = false ) {
  */
 function get_translations_for_domain( $domain ) {
 	global $l10n;
-	if ( isset( $l10n[ $domain ] ) ) {
+	if ( load_textdomain_just_in_time( $domain ) || isset( $l10n[ $domain ] ) ) {
 		return $l10n[ $domain ];
 	}
 
diff --git tests/phpunit/data/languages/plugins/internationalized-plugin-de_DE.mo tests/phpunit/data/languages/plugins/internationalized-plugin-de_DE.mo
new file mode 100644
index 0000000..e789268
Binary files /dev/null and tests/phpunit/data/languages/plugins/internationalized-plugin-de_DE.mo differ
diff --git tests/phpunit/data/languages/plugins/internationalized-plugin-de_DE.po tests/phpunit/data/languages/plugins/internationalized-plugin-de_DE.po
new file mode 100644
index 0000000..b1fff26
--- /dev/null
+++ tests/phpunit/data/languages/plugins/internationalized-plugin-de_DE.po
@@ -0,0 +1,23 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: \n"
+"POT-Creation-Date: 2015-12-31 16:31+0100\n"
+"PO-Revision-Date: 2016-04-28 18:50+0200\n"
+"Last-Translator: Pascal Birchler <pascal@required.ch>\n"
+"Language-Team: \n"
+"Language: de_DE\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"X-Generator: Poedit 1.8.4\n"
+"X-Poedit-Basepath: .\n"
+"Plural-Forms: nplurals=2; plural=(n != 1);\n"
+"X-Poedit-KeywordsList: __;_e;_x:1,2c;_ex:1,2c;_n:1,2;_nx:1,2,4c;_n_noop:1,2;"
+"_nx_noop:1,2,3c;esc_attr__;esc_html__;esc_attr_e;esc_html_e;esc_attr_x:1,2c;"
+"esc_html_x:1,2c\n"
+"X-Textdomain-Support: yes\n"
+"X-Poedit-SearchPath-0: .\n"
+
+#: internationalized-plugin.php:11
+msgid "This is a dummy plugin"
+msgstr "Das ist ein Dummy Plugin"
diff --git tests/phpunit/data/languages/themes/internationalized-theme-de_DE.mo tests/phpunit/data/languages/themes/internationalized-theme-de_DE.mo
new file mode 100644
index 0000000..cfd2b5a
Binary files /dev/null and tests/phpunit/data/languages/themes/internationalized-theme-de_DE.mo differ
diff --git tests/phpunit/data/languages/themes/internationalized-theme-de_DE.po tests/phpunit/data/languages/themes/internationalized-theme-de_DE.po
new file mode 100644
index 0000000..78c90e4
--- /dev/null
+++ tests/phpunit/data/languages/themes/internationalized-theme-de_DE.po
@@ -0,0 +1,23 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: \n"
+"POT-Creation-Date: 2015-12-31 16:38+0100\n"
+"PO-Revision-Date: 2016-04-28 18:50+0200\n"
+"Last-Translator: Pascal Birchler <pascal@required.ch>\n"
+"Language-Team: \n"
+"Language: de_DE\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"X-Generator: Poedit 1.8.4\n"
+"X-Poedit-Basepath: .\n"
+"Plural-Forms: nplurals=2; plural=(n != 1);\n"
+"X-Poedit-KeywordsList: __;_e;_x:1,2c;_ex:1,2c;_n:1,2;_nx:1,2,4c;_n_noop:1,2;"
+"_nx_noop:1,2,3c;esc_attr__;esc_html__;esc_attr_e;esc_html_e;esc_attr_x:1,2c;"
+"esc_html_x:1,2c\n"
+"X-Textdomain-Support: yes\n"
+"X-Poedit-SearchPath-0: .\n"
+
+#: functions.php:7
+msgid "This is a dummy theme"
+msgstr "Das ist ein Dummy Theme"
diff --git tests/phpunit/data/plugins/internationalized-plugin.php tests/phpunit/data/plugins/internationalized-plugin.php
new file mode 100644
index 0000000..4b56846
--- /dev/null
+++ tests/phpunit/data/plugins/internationalized-plugin.php
@@ -0,0 +1,12 @@
+<?php
+/*
+Plugin Name: Dummy Plugin
+Plugin URI: https://wordpress.org/
+Description: For testing purposes only.
+Version: 1.0.0
+Text Domain: internationalized-plugin
+*/
+
+function i18n_plugin_test() {
+	return __( 'This is a dummy plugin', 'internationalized-plugin' );
+}
diff --git tests/phpunit/data/themedir1/internationalized-theme/functions.php tests/phpunit/data/themedir1/internationalized-theme/functions.php
new file mode 100644
index 0000000..d836c4b
--- /dev/null
+++ tests/phpunit/data/themedir1/internationalized-theme/functions.php
@@ -0,0 +1,8 @@
+<?php
+/**
+ * Dummy theme.
+ */
+
+function i18n_theme_test() {
+	return __( 'This is a dummy theme', 'internationalized-theme' );
+}
diff --git tests/phpunit/data/themedir1/internationalized-theme/index.php tests/phpunit/data/themedir1/internationalized-theme/index.php
new file mode 100644
index 0000000..c674182
--- /dev/null
+++ tests/phpunit/data/themedir1/internationalized-theme/index.php
@@ -0,0 +1,4 @@
+<?php
+/**
+ * Dummy theme.
+ */
diff --git tests/phpunit/data/themedir1/internationalized-theme/style.css tests/phpunit/data/themedir1/internationalized-theme/style.css
new file mode 100644
index 0000000..ea472af
--- /dev/null
+++ tests/phpunit/data/themedir1/internationalized-theme/style.css
@@ -0,0 +1,7 @@
+/*
+Theme Name: Internationalized Theme
+Theme URI: https://wordpress.org/
+Description: For testing purposes only.
+Version: 1.0.0
+Text Domain: internationalized-theme
+*/
diff --git tests/phpunit/tests/l10n.php tests/phpunit/tests/l10n.php
index e5713da..7db9fa7 100644
--- tests/phpunit/tests/l10n.php
+++ tests/phpunit/tests/l10n.php
@@ -5,6 +5,39 @@
  * @group i18n
  */
 class Tests_L10n extends WP_UnitTestCase {
+	function setUp() {
+		parent::setUp();
+		$this->theme_root = DIR_TESTDATA . '/themedir1';
+
+		$this->orig_theme_dir = $GLOBALS['wp_theme_directories'];
+
+		// /themes is necessary as theme.php functions assume /themes is the root if there is only one root.
+		$GLOBALS['wp_theme_directories'] = array( WP_CONTENT_DIR . '/themes', $this->theme_root );
+
+		add_filter( 'theme_root', array( $this, '_theme_root' ) );
+		add_filter( 'stylesheet_root', array( $this, '_theme_root' ) );
+		add_filter( 'template_root', array( $this, '_theme_root' ) );
+		// clear caches
+		wp_clean_themes_cache();
+		unset( $GLOBALS['wp_themes'] );
+	}
+
+	function tearDown() {
+		$GLOBALS['wp_theme_directories'] = $this->orig_theme_dir;
+		remove_filter( 'theme_root', array( $this, '_theme_root' ) );
+		remove_filter( 'stylesheet_root', array( $this, '_theme_root' ) );
+		remove_filter( 'template_root', array( $this, '_theme_root' ) );
+		wp_clean_themes_cache();
+		unset( $GLOBALS['wp_themes'] );
+		parent::tearDown();
+	}
+
+	/**
+	 * Replace the normal theme root dir with our pre-made test dir
+	 */
+	function _theme_root() {
+		return $this->theme_root;
+	}
 
 	/**
 	 * @ticket 35961
@@ -44,6 +77,55 @@ class Tests_L10n extends WP_UnitTestCase {
 		$this->assertFalse( is_textdomain_loaded( 'wp-tests-domain' ) );
 	}
 
+	public function _set_locale_to_german() {
+		return 'de_DE';
+	}
+
+	/**
+	 * @ticket 34114
+	 */
+	public function test_load_plugin_textdomain_just_in_time() {
+		add_filter( 'locale', array( $this, '_set_locale_to_german' ) );
+
+		require_once DIR_TESTDATA . '/plugins/internationalized-plugin.php';
+
+		$is_textdomain_loaded_before = is_textdomain_loaded( 'internationalized-plugin' );
+		$expected_output             = i18n_plugin_test();
+		$is_textdomain_loaded_after  = is_textdomain_loaded( 'internationalized-plugin' );
+
+		remove_filter( 'locale', array( $this, '_set_locale_to_german' ) );
+
+		$this->assertFalse( $is_textdomain_loaded_before );
+		$this->assertSame( 'Das ist ein Dummy Plugin', $expected_output );
+		$this->assertTrue( $is_textdomain_loaded_after );
+	}
+
+	/**
+	 * @ticket 34114
+	 */
+	public function test_load_theme_textdomain_just_in_time() {
+		add_filter( 'locale', array( $this, '_set_locale_to_german' ) );
+
+		switch_theme( 'internationalized-theme' );
+
+		include_once( get_stylesheet_directory() . '/functions.php' );
+
+		$theme_name                  = wp_get_theme()->get( 'Name' );
+		$theme_validated             = validate_current_theme();
+		$is_textdomain_loaded_before = is_textdomain_loaded( 'internationalized-theme' );
+		$expected_output             = i18n_theme_test();
+
+		$is_textdomain_loaded_after = is_textdomain_loaded( 'internationalized-theme' );
+
+		remove_filter( 'locale', array( $this, '_set_locale_to_german' ) );
+
+		$this->assertSame( 'Internationalized Theme', $theme_name );
+		$this->assertTrue( $theme_validated );
+		$this->assertFalse( $is_textdomain_loaded_before );
+		$this->assertSame( 'Das ist ein Dummy Theme', $expected_output );
+		$this->assertTrue( $is_textdomain_loaded_after );
+	}
+
 	/**
 	 * @ticket 35073
 	 */
diff --git tests/phpunit/tests/theme/themeDir.php tests/phpunit/tests/theme/themeDir.php
index d5060f8..a6df7ba 100644
--- tests/phpunit/tests/theme/themeDir.php
+++ tests/phpunit/tests/theme/themeDir.php
@@ -159,6 +159,7 @@ class Tests_Theme_ThemeDir extends WP_UnitTestCase {
 			'My Subdir Theme',// theme in a subdirectory should work
 			'Page Template Theme', // theme with page templates for other test code
 			'Theme with Spaces in the Directory',
+			'Internationalized Theme',
 		);
 
 		sort($theme_names);