Update language_nl.h

Added these messages with Dutch translation
#define MSG_BED_Z
#define MSG_A_TRAVEL
#define MSG_HEATING_FAILED_LCD
#define MSG_ERR_REDUNDANT_TEMP
#define MSG_THERMAL_RUNAWAY
#define MSG_ERR_MAXTEMP
#define MSG_ERR_MINTEMP
#define MSG_ERR_MAXTEMP_BED
#define MSG_ERR_MINTEMP_BED
#define MSG_HEATING
#define MSG_HEATING_COMPLETE
#define MSG_BED_HEATING
#define MSG_BED_DONE