I put: they spoke klingon language
Why is taken as wrong?
You can say "Klingon" or "the Klingon language".
But we don't say "Klingon language" or "English language" or "French language" etc. in English without the definite article.
You can say "English is difficult" or "The English language is difficult" -- but "English language is difficult" is simply wrong.
Also, be careful about your capitalisation: the letters L and J and T do not exist in Klingon. Lu- has to be lu-; Jatlh has to be jatlh; TlhIngan has to be tlhIngan.
And in English, klingon has to be Klingon. It's a proper noun.