Index: install/install_data.sql
===================================================================
--- install/install_data.sql	(revision 13474)
+++ install/install_data.sql	(working copy)
@@ -34,6 +34,7 @@
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'UseModRewrite', '0', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_use_modrewrite', 'checkbox', '', '', 10.02, 0, 1, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'ModRewriteUrlEnding', '.html', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_ModRewriteUrlEnding', 'select', '', '=+,/=+/,.html=+.html', 10.021, 0, 0, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'ForceModRewriteUrlEnding', '0', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_ForceModRewriteUrlEnding', 'checkbox', '', NULL, 10.022, 0, 0, 'la_hint_ForceModRewriteUrlEnding');
+INSERT INTO ConfigurationValues VALUES(DEFAULT, 'UseContentLanguageNegotiation', '0', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_UseContentLanguageNegotiation', 'checkbox', '', '', 10.023, 0, 0, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'cms_DefaultDesign', '#default_design#', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_DefaultDesignTemplate', 'text', NULL, NULL, 10.03, 0, 0, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'ErrorTemplate', 'error_notfound', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_error_template', 'text', '', '', 10.04, 0, 0, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'NoPermissionTemplate', 'no_permission', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_nopermission_template', 'text', '', '', 10.05, 0, 0, NULL);
Index: install/upgrades.sql
===================================================================
--- install/upgrades.sql	(revision 13474)
+++ install/upgrades.sql	(working copy)
@@ -1810,3 +1810,5 @@
 UPDATE Phrase
 SET l<%PRIMARY_LANGUAGE%>_Translation = 'Enable SEO-friendly URLs mode (MOD-REWRITE)'
 WHERE Phrase = 'la_config_use_modrewrite' AND l<%PRIMARY_LANGUAGE%>_Translation = 'Use MOD REWRITE';
+
+INSERT INTO ConfigurationValues VALUES(DEFAULT, 'UseContentLanguageNegotiation', '0', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_UseContentLanguageNegotiation', 'checkbox', '', '', 10.023, 0, 0, NULL);
\ No newline at end of file
Index: kernel/utility/http_query.php
===================================================================
--- kernel/utility/http_query.php	(revision 13474)
+++ kernel/utility/http_query.php	(working copy)
@@ -263,8 +263,8 @@
 			}
 		}
 		else {
+			$this->Application->VerifyThemeId();
 			$this->Application->VerifyLanguageId();
-			$this->Application->VerifyThemeId();
 		}
 	}
 
Index: kernel/utility/unit_config_reader.php
===================================================================
--- kernel/utility/unit_config_reader.php	(revision 13474)
+++ kernel/utility/unit_config_reader.php	(working copy)
@@ -91,6 +91,7 @@
 
 			// output related
 			'UseModRewrite',
+			'UseContentLanguageNegotiation',
 			'UseOutputCompression',
 			'OutputCompressionLevel',
 			'Config_Server_Time',
Index: units/helpers/mod_rewrite_helper.php
===================================================================
--- units/helpers/mod_rewrite_helper.php	(revision 13474)
+++ units/helpers/mod_rewrite_helper.php	(working copy)
@@ -266,9 +266,9 @@
 
 		function InitAll()
 		{
+			$this->Application->VerifyThemeId();
 			$this->Application->VerifyLanguageId();
 			$this->Application->Phrases->Init('phrases');
-			$this->Application->VerifyThemeId();
 		}
 
 		/**
Index: units/languages/languages_item.php
===================================================================
--- units/languages/languages_item.php	(revision 13474)
+++ units/languages/languages_item.php	(working copy)
@@ -56,17 +56,128 @@
 			return number_format($number, $precision, $this->GetDBField('DecimalPoint'), $this->GetDBField('ThousandSep'));
 		}
 
+		/**
+		 * Returns language id based on HTTP header "Accept-Language"
+		 *
+		 * @return int
+		 */
+		function processAcceptLanguage()
+		{
+			if (!array_key_exists('HTTP_ACCEPT_LANGUAGE', $_SERVER) || !$_SERVER['HTTP_ACCEPT_LANGUAGE']) {
+				return false;
+			}
+
+			$accepted_languages = Array ();
+			$language_string = explode(',', strtolower($_SERVER['HTTP_ACCEPT_LANGUAGE']) );
+
+			foreach ($language_string as $mixed_language) {
+				if (strpos($mixed_language, ';') !== false) {
+					list ($language_locale, $language_quality) = explode(';', $mixed_language);
+					$language_quality = (float)substr($language_quality, 2); // format "q=4.45"
+				}
+				else {
+					$language_locale = $mixed_language;
+					$language_quality = 1.00;
+				}
+
+				$accepted_languages[ trim($language_locale) ] = trim($language_quality);
+			}
+
+			arsort($accepted_languages, SORT_NUMERIC);
+		
+			foreach ($accepted_languages as $language_locale => $language_quality) {
+				$language_id = $this->getAvailableLanguage($language_locale);
+
+				if ($language_id) {
+					return $language_id;
+				}
+			}
+			
+			return false;
+		}
+
+		/**
+		 * Returns language ID based on given locale
+		 *
+		 * @param string $locale
+		 * @return int
+		 */
+		function getAvailableLanguage($locale)
+		{
+			static $available_languages;
+			
+			$cache_key = 'available_languages[%LangSerial%]';
+			$available_languages = $this->Application->getCache($cache_key);
+
+			if ($available_languages === false) {
+				$this->Conn->nextQueryCachable = true;
+				$sql = 'SELECT LanguageId, LOWER(Locale) AS Locale
+						FROM ' . TABLE_PREFIX . 'Language
+						WHERE Enabled = 1';
+				$available_languages = $this->Conn->GetCol($sql, 'Locale');
+				$this->Application->setCache($cache_key, $available_languages);
+			}
+
+			if (strpos($locale, '-') !== false) {
+				// exact language match requested
+				return array_key_exists($locale, $available_languages) ? $available_languages[$locale] : false;
+			}
+
+			// partial (like "en" matches "en-GB" and "en-US") language match required
+			foreach ($available_languages as $language_code => $language_id) {
+				list ($language_code, ) = explode('-', $language_code);
+
+				if ($locale == $language_code) {
+					return $language_id;
+				}
+			}
+			
+			return false;
+		}
+
+		/**
+		 * Checks, that url is empty
+		 *
+		 * @return bool
+		 */
+		function isEmptyUrl()
+		{
+			if ($this->Application->RewriteURLs()) {
+				return !$this->Application->GetVar('_mod_rw_url_');
+			}
+
+			return !count($this->Application->HttpQuery->Get);
+		}
+
 		function Load($id, $id_field_name=null)
 		{
 			$default = false;
 			if ($id == 'default') {
-				$id = 1;
-				$id_field_name = 'PrimaryLang';
 				$default = true;
+				$res = parent::Load(1, 'PrimaryLang', true);
+
+				if (
+					!$this->Application->isAdmin && $this->isEmptyUrl() &&
+					$this->Application->ConfigValue('UseContentLanguageNegotiation')
+				) {
+					$language_id = $this->processAcceptLanguage();
+
+					if ($language_id != $this->GetID()) {
+						// redirect to same page with found language
+						$url_params = Array (
+							'm_cat_id' => 0, 'm_cat_page' => 1,
+							'm_lang' => $language_id, 'm_opener' => 's',
+							'pass' => 'm'
+						);
+
+						$this->Application->Redirect('', $url_params);
+					}
+				}
 			}
+			else {
+				$res = parent::Load($id, $id_field_name, true);
+			}
 
-			$res = parent::Load($id, $id_field_name, true);
-
 			if ($default) {
 				if (!$res) {
 					if ($this->Application->isAdmin) {
@@ -86,6 +197,7 @@
 				$this->Application->SetVar('lang.current_id', $this->GetID() );
 				$this->Application->SetVar('m_lang', $this->GetID() );
 			}
+
 			return $res;
 		}
 	}
\ No newline at end of file