How to create new namespace

I want to translate the Country Page of Luxembourg into the luxembourgish language, but this name space does not yet exist. I´ve read, the namespaces are created by a parser function of MediWiki. I don´t really understand that.
Can onyone explain to me, how I can create this new namespace LB:Luxembourgish, so that it appears on my “missing languages” bar?

Thx in advance

Do you know ?

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