Should diacritics be kept when citing a paper written in an abundant diacritics Latin-based script language?
Nowadays, every sufficiently recent typesetting system is able to deal with diacritics. When submitting to journals, the real problem are the publishers' typesetters, who are -- at least in my experience -- frequently careless in copying letters with diacritics. 

So, my advice is: keep the diacritics, as the style guides and common sense suggest, but check carefully the proofs received from the publisher.