diff --git a/Firmware/Marlin_main.cpp b/Firmware/Marlin_main.cpp
index e984f821..c98a42ac 100644
--- a/Firmware/Marlin_main.cpp
+++ b/Firmware/Marlin_main.cpp
@@ -1113,7 +1113,7 @@ uint8_t lang_xflash_enum_codes(uint16_t* codes)
 		printf_P(_n(" _lt_count        = 0x%04x (%d)\n"), header.count, header.count);
 		printf_P(_n(" _lt_chsum        = 0x%04x\n"), header.checksum);
 		printf_P(_n(" _lt_code         = 0x%04x (%c%c)\n"), header.code, header.code >> 8, header.code & 0xff);
-		printf_P(_n(" _lt_resv1        = 0x%08lx\n"), header.reserved1);
+		printf_P(_n(" _lt_sign         = 0x%08lx\n"), header.signature);
 
 		addr += header.size;
 		codes[count] = header.code;
@@ -1190,7 +1190,7 @@ void setup()
 #ifdef DEBUG_SEC_LANG
 	lang_table_header_t header;
 	uint32_t src_addr = 0x00000;
-	if (lang_get_header(3, &header, &src_addr))
+	if (lang_get_header(1, &header, &src_addr))
 	{
 //this is comparsion of some printing-methods regarding to flash space usage and code size/readability
 #define LT_PRINT_TEST 2
@@ -1207,7 +1207,7 @@ void setup()
 		printf_P(_n(" _lt_count = 0x%04x (%d)\n"), header.count, header.count);
 		printf_P(_n(" _lt_chsum = 0x%04x\n"), header.checksum);
 		printf_P(_n(" _lt_code  = 0x%04x (%c%c)\n"), header.code, header.code >> 8, header.code & 0xff);
-		printf_P(_n(" _lt_resv1 = 0x%08lx\n"), header.reserved1);
+		printf_P(_n(" _lt_sign = 0x%08lx\n"), header.signature);
 #elif (LT_PRINT_TEST==2) //optimized printf
 		printf_P(
 		 _n(
@@ -1225,7 +1225,7 @@ void setup()
 		 header.count, header.count,
 		 header.checksum,
 		 header.code, header.code >> 8, header.code & 0xff,
-		 header.reserved1
+		 header.signature
 		);
 #elif (LT_PRINT_TEST==3) //arduino print/println (leading zeros not solved)
 		MYSERIAL.print(" _src_addr = 0x");
@@ -1252,7 +1252,7 @@ void setup()
 		MYSERIAL.print((char)(header.code & 0xff), 0);
 		MYSERIAL.println(")");
 		MYSERIAL.print(" _lt_resv1 = 0x");
-		MYSERIAL.println(header.reserved1, 16);
+		MYSERIAL.println(header.signature, 16);
 #endif //(LT_PRINT_TEST==)
 #undef LT_PRINT_TEST
 
@@ -1265,7 +1265,22 @@ void setup()
 			if ((i % 16) == 15) putchar('\n');
 		}
 #endif
-#if 1
+		uint16_t sum = 0;
+		for (uint16_t i = 0; i < header.size; i++)
+			sum += (uint16_t)pgm_read_byte((uint8_t*)(_SEC_LANG_TABLE + i)) << ((i & 1)?0:8);
+		printf_P(_n("_SEC_LANG_TABLE checksum = %04x\n"), sum);
+		sum -= header.checksum; //subtract checksum
+		printf_P(_n("_SEC_LANG_TABLE checksum = %04x\n"), sum);
+		sum = (sum >> 8) | ((sum & 0xff) << 8); //swap bytes
+		if (sum == header.checksum)
+			printf_P(_n("Checksum OK\n"), sum);
+		else
+			printf_P(_n("Checksum NG\n"), sum);
+	}
+	else
+		printf_P(_n("lang_get_header failed!\n"));
+
+#if 0
 		for (uint16_t i = 0; i < 1024*10; i++)
 		{
 			if ((i % 16) == 0) printf_P(_n("%04x:"), _SEC_LANG_TABLE+i);
@@ -1273,10 +1288,6 @@ void setup()
 			if ((i % 16) == 15) putchar('\n');
 		}
 #endif
-	}
-	else
-		printf_P(_n("lang_get_header failed!\n"));
-
 
 #if 0
 	SERIAL_ECHOLN("Reading eeprom from 0 to 100: start");
diff --git a/Firmware/config.h b/Firmware/config.h
index a9172429..85687a4b 100644
--- a/Firmware/config.h
+++ b/Firmware/config.h
@@ -29,7 +29,7 @@
 //LANG - Multi-language support
 //#define LANG_MODE               0 // primary language only
 #define LANG_MODE               1 // sec. language support
-#define LANG_SIZE_RESERVED 0x2700 // reserved space for secondary language (~10kb)
+#define LANG_SIZE_RESERVED 0x2400 // reserved space for secondary language (~10kb)
 //#define LANG_SIZE_RESERVED 0x1ef8 // reserved space for secondary language (~10kb)
 
 
diff --git a/Firmware/language.c b/Firmware/language.c
index acac7d1b..b79da8ff 100644
--- a/Firmware/language.c
+++ b/Firmware/language.c
@@ -28,6 +28,9 @@ uint8_t lang_is_selected(void) { return 1; }
 //reserved xx kbytes for secondary language table
 const char _SEC_LANG[LANG_SIZE_RESERVED] PROGMEM_I2 = "_SEC_LANG";
 
+//primary language signature
+const uint32_t _PRI_LANG_SIGNATURE[1] __attribute__((section(".progmem0"))) = {0xffffffff};
+
 //lang_table pointer
 lang_table_t* lang_table = 0;
 
@@ -56,8 +59,12 @@ uint8_t lang_select(uint8_t lang)
 	{
 		if (pgm_read_dword(((uint32_t*)_SEC_LANG_TABLE)) == LANG_MAGIC) //magic valid
 		{
-			lang_table = _SEC_LANG_TABLE; // set table pointer
-			lang_selected = lang; // set language id
+			if (lang_check(_SEC_LANG_TABLE))
+				if (pgm_read_dword(((uint32_t*)(_SEC_LANG_TABLE + 12))) == pgm_read_dword(((uint32_t*)(_PRI_LANG_SIGNATURE)))) //signature valid
+				{
+					lang_table = _SEC_LANG_TABLE; // set table pointer
+					lang_selected = lang; // set language id
+				}
 		}
 	}
 #else //W25X20CL
@@ -70,6 +77,18 @@ uint8_t lang_select(uint8_t lang)
 	return 0;
 }
 
+uint8_t lang_check(uint16_t addr)
+{
+	uint16_t sum = 0;
+	uint16_t size = pgm_read_word((uint16_t*)(addr + 4));
+	uint16_t lt_sum = pgm_read_word((uint16_t*)(addr + 8));
+	uint16_t i; for (i = 0; i < size; i++)
+		sum += (uint16_t)pgm_read_byte((uint8_t*)(addr + i)) << ((i & 1)?0:8);
+	sum -= lt_sum; //subtract checksum
+	sum = (sum >> 8) | ((sum & 0xff) << 8); //swap bytes
+	return (sum == lt_sum);
+}
+
 uint8_t lang_get_count()
 {
 #ifdef W25X20CL
@@ -98,7 +117,7 @@ uint8_t lang_get_header(uint8_t lang, lang_table_header_t* header, uint32_t* off
 		uint16_t ui = _SEC_LANG_TABLE; //table pointer
 		memcpy_P(header, ui, sizeof(lang_table_header_t)); //read table header from progmem
 		if (offset) *offset = ui;
-		return (header == LANG_MAGIC)?1:0; //return 1 if magic valid
+		return (header->magic == LANG_MAGIC)?1:0; //return 1 if magic valid
 	}
 	W25X20CL_SPI_ENTER();
 	uint32_t addr = 0x00000; //start of xflash
diff --git a/Firmware/language.h b/Firmware/language.h
index 359bc083..509b0bc6 100644
--- a/Firmware/language.h
+++ b/Firmware/language.h
@@ -52,7 +52,7 @@ typedef struct
 	uint16_t count;      //+6
 	uint16_t checksum;   //+8
 	uint16_t code;       //+10
-	uint32_t reserved1;  //+12
+	uint32_t signature;  //+12
 } lang_table_header_t;
 
 //lang_table_t structure - (size= 16byte + 2*count)
@@ -98,10 +98,13 @@ extern uint8_t lang_selected;
 extern const char _SEC_LANG[LANG_SIZE_RESERVED];
 extern const char* lang_get_translation(const char* s);
 #define _SEC_LANG_TABLE ((((uint16_t)&_SEC_LANG) + 0x00ff) & 0xff00)
+//extern const uint32_t _PRI_LANG_SIGNATURE;
 #endif //(LANG_MODE != 0)
 
 //selects language, eeprom is updated in case of success
 extern uint8_t lang_select(uint8_t lang);
+//performs checksum test of secondary language data
+extern uint8_t lang_check(uint16_t addr);
 //returns total number of languages (primary + all in xflash)
 extern uint8_t lang_get_count(void);
 //reads lang table header and offset in xflash or progmem
diff --git a/Firmware/optiboot_w25x20cl.cpp b/Firmware/optiboot_w25x20cl.cpp
index ce50323f..cc6c66cd 100644
--- a/Firmware/optiboot_w25x20cl.cpp
+++ b/Firmware/optiboot_w25x20cl.cpp
@@ -5,6 +5,7 @@
 #include "Marlin.h"
 #include "w25x20cl.h"
 #include "stk500.h"
+#include "bootapp.h"
 
 #define OPTIBOOT_MAJVER 6
 #define OPTIBOOT_CUSTOMVER 0
@@ -98,6 +99,7 @@ extern struct block_t *block_buffer;
 
 void optiboot_w25x20cl_enter()
 {
+  if (boot_app_flags & BOOT_APP_FLG_USER0) return;
   uint8_t ch;
   uint8_t rampz = 0;
   register uint16_t address = 0;
diff --git a/Firmware/ultralcd.cpp b/Firmware/ultralcd.cpp
index b4fbe6f7..74e0b349 100644
--- a/Firmware/ultralcd.cpp
+++ b/Firmware/ultralcd.cpp
@@ -3682,11 +3682,18 @@ void lcd_set_progress() {
 static void lcd_language_menu()
 {
 	START_MENU();
-	if (lang_is_selected()) MENU_ITEM(back, _T(MSG_SETTINGS), 0);
-	MENU_ITEM(setlang, lang_get_name_by_code(lang_get_code(0)), 0);
-//	MENU_ITEM(setlang, lang_get_name_by_code(lang_get_code(1)), 1);
-	for (int i = 2; i < lang_get_count(); i++) //skip seconday language - solved in lang_select
-		MENU_ITEM(setlang, lang_get_name_by_code(lang_get_code(i)), i);
+	if (lang_is_selected()) MENU_ITEM(back, _T(MSG_SETTINGS), 0); //
+	MENU_ITEM(setlang, lang_get_name_by_code(lang_get_code(0)), 0); //primary language
+	uint8_t cnt = lang_get_count();
+#ifdef W25X20CL
+	if (cnt == 2) //display secondary language in case of clear xflash 
+		MENU_ITEM(setlang, lang_get_name_by_code(lang_get_code(1)), 1);
+	else
+		for (int i = 2; i < cnt; i++) //skip seconday language - solved in lang_select (MK3)
+#else //W25X20CL
+		for (int i = 1; i < cnt; i++) //all seconday languages (MK2/25)
+#endif //W25X20CL
+			MENU_ITEM(setlang, lang_get_name_by_code(lang_get_code(i)), i);
 	END_MENU();
 }
 #endif //(LANG_MODE != 0)
diff --git a/lang/make_lang.sh b/lang/make_lang.sh
index 261857e4..d34aad3f 100644
--- a/lang/make_lang.sh
+++ b/lang/make_lang.sh
@@ -86,7 +86,7 @@ lt_count=$(grep -c '^' lang_$LANG.txt)
 lt_data_size=$(wc -c lang_$LANG.dat | cut -f1 -d' ')
 lt_offs_size=$((2 * $lt_count))
 lt_size=$((16 + $lt_offs_size + $lt_data_size))
-lt_chsum=1
+lt_chsum=0
 lt_code='\xff\xff'
 lt_resv1='\xff\xff\xff\xff'
 
@@ -138,6 +138,11 @@ echo -n "  writing text data ($lt_data_size bytes)..." >&2
 dd if=./lang_$LANG.dat of=./lang_$LANG.bin bs=1 count=$lt_data_size seek=$((16 + $lt_offs_size)) conv=notrunc 2>/dev/null
 echo "OK" >&2
 
+#calculate and update checksum
+lt_chsum=$(cat lang_$LANG.bin | xxd | cut -c11-49 | tr ' ' "\n" | sed '/^$/d' | awk 'BEGIN { sum = 0; } { sum += strtonum("0x"$1); if (sum > 0xffff) sum -= 0x10000; } END { printf("%x\n", sum); }')
+/bin/echo -n -e $(echo -n $((0x$lt_chsum)) | awk "$awk_ui16") |\
+ dd of=lang_$LANG.bin bs=1 count=2 seek=8 conv=notrunc 2>/dev/null
+
 echo " lang_table details:" >&2
 echo "  lt_count = $lt_count" >&2
 echo "  lt_size  = $lt_size" >&2
diff --git a/lang/update_lang.sh b/lang/update_lang.sh
index 6acf9f8f..b28081b6 100644
--- a/lang/update_lang.sh
+++ b/lang/update_lang.sh
@@ -40,9 +40,15 @@ echo -n " checking symbols..." >&2
 #find symbol _SEC_LANG in section '.text'
 sec_lang=$(cat text.sym | grep -E "\b_SEC_LANG\b")
 if [ -z "$sec_lang" ]; then echo "NG!\n  symbol _SEC_LANG not found!" >&2; finish 1; fi
+#find symbol _PRI_LANG_SIGNATURE in section '.text'
+pri_lang=$(cat text.sym | grep -E "\b_PRI_LANG_SIGNATURE\b")
+if [ -z "$pri_lang" ]; then echo "NG!\n  symbol _PRI_LANG_SIGNATURE not found!" >&2; finish 1; fi
 echo "OK" >&2
 
 echo " calculating vars:" >&2
+#get pri_lang addres
+pri_lang_addr='0x'$(echo $pri_lang | cut -f1 -d' ')
+echo "  pri_lang_addr   =$pri_lang_addr" >&2
 #get addres and size
 sec_lang_addr='0x'$(echo $sec_lang | cut -f1 -d' ')
 sec_lang_size='0x'$(echo $sec_lang | cut -f2 -d' ')
@@ -62,6 +68,7 @@ printf "  lang_file_size  =0x%04x (=%d bytes)\n" $lang_file_size $lang_file_size
 if [ $lang_file_size -gt $lang_table_size ]; then echo "Lanaguage binary file size too big!" >&2; finish 1; fi
 
 echo "updating 'firmware.bin'..." >&2
+
 dd if=lang_$LANG.bin of=firmware.bin bs=1 seek=$lang_table_addr conv=notrunc 2>/dev/null
 
 #convert bin to hex