It actually isn't (or very unnatural to say the least...especially at the beginning of the sentence). :) "where" is actually pretty vague in English and could have the meanings or "nerede," "nereye," or "neresi" in Turkish.
"nereye" by itself could mean "where to?" "to where" is just kind of an unnatural sounding long stretch though, although if a few other native speakers agree with you, I guess I could budge.
You're right that "to where" is very unnatural in English. Some people still believe the old (discredited) rule about not ending sentences in prepositions, but "Where are you traveling to?" is perfectly okay. English used to have "whither" and "whence" for "where to" and "where from," but we've left them behind. Pity.
"To where is the trip?" English speakers of a certain age will still remember being taught that this was the correct way to form a sentence like this, particularly for use in formal situations/writings. Although no-one really speaks like this today, we do still hear it from time to time in some of the many period dramas that have become so popular on television. "To where is the trip?" and "Where is the trip to?" essentially mean the same thing, but I agree, the former does sound stilted. I guess it boils down to where and when you went to school (and how many languages you've learnt since), as to which form you choose first. Just my thruppeny bit's worth :)
You can read the comments above. This is an idiomatic expression in which "you" is understood. Its English equivalent is "Where are you traveling?" or "Where are you traveling to?" You could also say "Where is the trip to?" if someone said they were taking a trip soon -- "yolculuk" is a noun -- and I believe Duolingo accepts that. "To where travel" isn't an English question. If someone was taking trips frequently, you might ask, "Where do you travel to?" (or just "Where do you go?") It must be difficult to learn a language through the medium of English when English is not your first language, but that's how Duolingo works.