Sorry, I realise I was unclear - I meant when given the Russian Сергей/Сергея, why doesn't it accept 'Sergei' as the English equivalent (when asking for the English) From your answer though, I now realise that it probably sticks completely to one standard transliteration system, even for names (but I automatically wrote 'Sergei' not 'Sergey', as I have two friends who write 'Sergei' when writing in English).
in spite of my having informed DUO twice about the problem of chut-chut, they still haven't made the necessary correction ! I cannot proceed and finish the tree because if I write chut'-chut' with an apostrophe like DUO does, they say it is wrong ! and if I write it without the apostrophe they say it is also wrong !! I am stuck here !