We don't. We say "one of each country's leader", which means that we have chosen as a clue either of the two leaders that are possible clues for most of these countries. She's "Theresa", by the way, with an "h".
Quiz updated. Nearly all of the leaders have changed in the past five years, with the notable exceptions of Putin and Belarus's election-rigging president.
Same reasoning as stated in our earlier answers. We've just updated the quiz and are astonished at the huge turnover of leaders. Some, like Switzerland with its one-year rotating presidency, make sense, but it looks like the shelf-life of leaders is pretty short. Who would want to be a politician?