After about an hour of trying different search strings, I've read umpteen pages that all assume the person's income is from Turkish sources. One of them hints that a pension from another country is exempt. None of them had anything to say about interest, social security, or capital gains from another country. Nor static assets, i.e., the account balances and stock shares owned.
I don't live in Turkey now—I'm trying to estimate expenses if I move there. Is there a source a non-lawyer can consult on this? Not looking for info on IRS taxes—I'm already familiar with that.
I've been checking into a lot of countries and running into the same problem. For reasons I don't understand, even pages about retiring in another country seem to have this problem.