"We are going somewhere. Mark knows where."
Translation:Ми їдемо кудись. Марк знає куди.
Your explanation of the two words matches the dictionary hints however, "somewhere" could very easily be capitalized thus making it a proper noun or the name of a ????. This would make Somewhere a destination. English is my native lanuage and i have travelled throught most of the English speaking countries and have yet to come across an instance of "somewhere" being a direction. Often if someone says "I am going somewhere" it is because they desire to keep the destination from the other person.
In conclusion, the Ukrainian sentence does not match the English unless кудись is an elusive answer rather than specific.