language.h: update documentation for LANGUAGE_INCLUDE

language.h: update documentation for LANGUAGE_INCLUDE

@thinkyhead What do you think? I think there should be a warning like
this.

Marlin/language.h

 // kana     Japanese
 // kana_utf Japanese
 
+// fallback if no language is set, don't change
 #ifndef LANGUAGE_INCLUDE
-  // pick your default from the list above, individual language ist set in LCD section of Configuration.h
   #define LANGUAGE_INCLUDE GENERATE_LANGUAGE_INCLUDE(en)
 #endif