Translation:They described the woman who stole their motorcycle.
I'm not sure exactly what you're asking, but both of those sentences could be "real" sentences in English.
The first one would mean that "they" did not have a motorcycle. But some woman stole one, and she gave it to "them." And then "they" described, to someone else, the woman who stole the motorcycle and kindly gave it to "them."
In the second sentence, "they" described to some other people ("them") the woman who stole the motorcycle. But in this case, the woman may or may not have given the motorcycle to the people who are describing her.
Hope that was something like what you were looking for, and also makes some sense... :-)
Thank you for the explanation. Originally I thought that the both sentences have the same mean as original sentence in czech ("Popsali ženu, která jim ukradla motorku."). After your explanation I think that
They described the woman who stole their motorcycle. = Popsali ženu, která ukradla JEJICH motorku.
Popsali ženu, která JIM ukradla motorku. = They described the woman who stole them a motorcycle. - Exactly like you explained above.
The second sentence in czech should have two meanings. 1) "The woman who stole their motorcycle" 2) "The woman who stole them a motorcycle" (The motorcycle for them).
It is just how English works, you can't translate languages word by word, you must respect their natural phrasings. Czech uses "On mi ukradl kolo." (mi in dative) and English uses "He stole my bike." (my as a possessive pronoun). In Czech you use the possessive pronoun as well: "On ukradl moje kolo.".
I may be misunderstanding your comment, but "They described the woman who stole them the motorcycle" is not normally correct in English in the meaning of this sentence. It might be used by some with the meaning of, "They described the woman who stole the motorcycle FOR THEM," in other circumstances.