#include "language_en.h" before the national language in language.h

+ May decrease the workload for feature developers. They may omit to
update all the language files but "language_en.h". When loading _en
first, all new features give some display, but the new in English.

- May throw some additional warnings about redefinition during compile.

Marlin/language.h

@@ -222,6 +222,7 @@
 #define TESTSTRING360 "\360\361\362\363\364\365\366\367\370\371\372\373\374\375\376\377"
 */
 
+#include "language_en.h"
 #include LANGUAGE_INCLUDE
 
 #endif //__LANGUAGE_H