Using the new language implementation wasn't behaving as expected.

No matter what LANGUAGE_CHOICE was set to it would always take the en branch.
I revamped it a bit to use a macro to create the language include file name. This required a slight tweak to the RU declaration since it's special-cased in a couple places in code.

This is an attempt to pull my changes from #1222 over.

Marlin/dogm_lcd_implementation.h

@@ -41,7 +41,7 @@
 
 /* Russian language not supported yet, needs custom font
 
-#if LANGUAGE_CHOICE == ru
+#ifdef LANGUAGE_RU
 #include "LiquidCrystalRus.h"
 #define LCD_CLASS LiquidCrystalRus
 #else

Marlin/language.h

@@ -1,10 +1,14 @@
 #ifndef LANGUAGE_H
 #define LANGUAGE_H
 
+#define LANGUAGE_CONCAT(M)       #M
+#define GENERATE_LANGUAGE_INCLUDE(M)  LANGUAGE_CONCAT(language_##M.h)
+
+
 // NOTE: IF YOU CHANGE LANGUAGE FILES OR MERGE A FILE WITH CHANGES
 //
 //   ==> ALWAYS TRY TO COMPILE MARLIN WITH/WITHOUT "ULTIPANEL" / "ULTRALCD" / "SDSUPPORT" #define IN "Configuration.h"
-//   ==> ALSO TRY ALL AVAILABLE "LANGUAGE_CHOICE" OPTIONS
+//   ==> ALSO TRY ALL AVAILABLE LANGUAGE OPTIONS
 
 // Languages
 // en English
@@ -21,12 +25,9 @@
 // ca Catalan
 // eu Basque-Euskera
 
-#ifndef LANGUAGE_CHOICE
+#ifndef LANGUAGE_INCLUDE
-	#define LANGUAGE_CHOICE en  // Pick your language from the list above
+  // pick your language from the list above
-#endif
+  #define LANGUAGE_INCLUDE GENERATE_LANGUAGE_INCLUDE(en)
-
-#if LANGUAGE_CHOICE >= 1
-  #error "Languages now uses ISO language codes instead of numbers."
 #endif
 
 #define PROTOCOL_VERSION "1.0"
@@ -152,32 +153,6 @@
 
 // LCD Menu Messages
 
-#if   LANGUAGE_CHOICE == en // English
+#include LANGUAGE_INCLUDE
-  #include "language_en.h"
-#elif LANGUAGE_CHOICE == pl // Polish
-  #include "language_pl.h"
-#elif LANGUAGE_CHOICE == fr // French
-  #include "language_fr.h"
-#elif LANGUAGE_CHOICE == de // German
-  #include "language_de.h"
-#elif LANGUAGE_CHOICE == es // Spanish
-  #include "language_es.h"
-#elif LANGUAGE_CHOICE == ru // Russian
-  #include "language_ru.h"
-#elif LANGUAGE_CHOICE == it // Italian
-  #include "language_it.h"
-#elif LANGUAGE_CHOICE == pt // Portuguese
-  #include "language_pt.h"
-#elif LANGUAGE_CHOICE == fi // Finnish
-  #include "language_fi.h"
-#elif LANGUAGE_CHOICE == an // Aragonese
-  #include "language_an.h"
-#elif LANGUAGE_CHOICE == nl // Dutch
-  #include "language_nl.h"
-#elif LANGUAGE_CHOICE == ca // Catalan
-  #include "language_ca.h"
-#elif LANGUAGE_CHOICE == eu // Basque-Euskera
-  #include "language_eu.h"
-#endif
 
 #endif //__LANGUAGE_H

Marlin/language_ru.h

@@ -8,6 +8,8 @@
 #ifndef LANGUAGE_RU_H
 #define LANGUAGE_RU_H
 
+#define LANGUAGE_RU
+
 #define WELCOME_MSG                         MACHINE_NAME "Готов."
 #define MSG_SD_INSERTED                     "Карта вставлена"
 #define MSG_SD_REMOVED                      "Карта извлечена"

Marlin/ultralcd_implementation_hitachi_HD44780.h

@@ -199,7 +199,7 @@ extern volatile uint16_t buttons;  //an extended version of the last checked but
 
 #else
   // Standard directly connected LCD implementations
-  #if LANGUAGE_CHOICE == ru
+  #ifdef LANGUAGE_RU
     #include "LiquidCrystalRus.h"
     #define LCD_CLASS LiquidCrystalRus
   #else