That was also a hitch with the German's Enigma usage in WW2. Their rock-solid belief that it was unbreakable led operators to have standard message formats - eg. weather reports always started by saying "Wetter" (that's German for "Weather", not that it was always raining ). When the breakers realised this, they simply searched for which arrangement of the dials would give this result. Hey presto: the code keys for that day !

Later, the Germans introduced code abbreviations to break up the language (eg. a weather report could be denoted by "AB", raining could be "XY", so what's sent out would be the encyptment of "ABXY"). This both removed language sequences, and also shortened the messages. A lot tougher to break, but they managed it.