You answer in the affirmative by repeating the verb form , if this is what is being questioned:
Oletteko te suomalainen? Are you Finnish?
Olen. Yes.
If the yes/no-question suffix –ko/-kö was added to any word other than the verb, you say niin; it is more polite to repeat the question word as well:
Ruotsalainenko sä olet? Are you Swedish?
Niin, ruotsalainen. Yes, I am.
The second example seems to translate as “Are you really Swedish?”. Have I got the sense correct?
Also, I have not heard this kind of question so far in everyday speech, where the –ko is added to something other than a verb. Is this kind of form common?