"체코어는 어렵다고 들었어요."
Translation:I heard that Czech is difficult.
은/는 is the topic particle, as others have stated, but it would be okay to exchange it for the subject particle 이/가 here, since Czech is also the subject of the sentence. The given sentence sounds like it would be used in the middle of a conversation, after talking about some other related thing, in order to shift the focus to the topic of the Czech language. You can think of it as saying, "While we're on the subject of languages (or the Czech Republic), I heard that Czech is difficult."
What you entered has the same essential meaning as the given sentence, but ordered differently, so that's why it was marked wrong. To compare:
"I heard that Czech is difficult" = 체코어는 어렵다고 들었어요. "(That) Czech is difficult" is the complement clause, so it comes first, before the conjunction ~고.
"I heard that Czech is a difficult language" = 체코어는 어려운 언어다고 들었어요. "(That) Czech is a difficult language" is the complement, so it comes before the conjunction ~고. The copula stem 이 in 언어이다고 is usually dropped.
Right, 체코어 can be translated as either 'Czech' or 'the Czech language.' But this has more to do with your sentence having a 'noun is noun phrase' predicate rather than 'noun is adjective'. "Czech/the Czech language is difficult" is 'noun is adjective', and 어렵다 means 'to be difficult'; "Czech/the Czech language is a difficult language" is 'noun is noun phrase,' and 어려운 언어다 means 'to be a difficult language.'