Genitive in Norwegian

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In Norwegian, the genitive is normally formed by word + s, except in cases where word ends in s, x or z, where the genitive is word + ' (apostrophe). (Regardless of pronunciation – even French names with silent s get the apostrophe.) In some messages, like Mobile-frontend-talk-back-to-userpage ("Return to the user page of "{{GENDER:$1|$1}}"."), you need to use the genitive of $1, where $1 is e.g. a username. Is there a way to use some function, maybe {{GRAMMAR:}}, to achieve this? We have ways of rewriting such messages, but they will often sound more awkward.