Alternatively, it tries to get you to say "I understand Klingon languages." Is the plurality really necessary?
Do you mean as an alternative to "I understand Klingon language"?
Well, the best alternative to that would be "I understand the Klingon language" -- with article "the" before the countable noun "language".
If you entered "I understand Klingon language", I'm guessing that it proposed "Klingon languages" because it considered that a smaller change (keeping the same number of words, just adding a letter, versus adding a word while keeping the others unchanged).
"I understand Klingon language" sounds wrong to me much the same way that "I live in free country" or "I am wearing warm shirt" or "I have loving father" are wrong.
I would accept "I understand Klingon language" as correct English, though unusual. Imagine you're reading Tolkien and you come across a sentence, "I understand bear-language." This would make perfect sense if discussing talking bears.