
Is it correct 'an misfortune' instead of 'a misfortune' ?

It would need to be "a misfortune".
("a" before words starting with a consonant).
However, it may be better to leave out the "a" and say "Misfortune never comes alone" as the sentence is talking of misfortune in general, not a particular one.
(my opinion only and I realise the German says "a misfortune")
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