Translation:The Klingon does not understand Federation Standard.
I see a report from the same time as your comment (possibly from you?) that "The Klingon does not understood English" should be accepted.
The problem in that sentence is not "English"; the problem is the verb "does not understood".
(I'm guessing this was a multiple-choice exercise? Where the system picked one of the correct answers, "The Klingon does not understand English", and replaced the word "understand" with "understood" to create a distractor sentence.)