Thanks everyone who replied to my post.
I have proposed to change $wgSitename to "ウィキペディア" in Japanese Wikipedia. A few users oppose itself. The half of users support changing not the prefix of project namespace but only site's name, while others support changes of both $wgSitename and project namespace's prefix.
Leaving the door open to further change, can we set $wgSitename = "ウィ キペディア" and $wgMetaNamespace = "Wikipedia"?
Sorry for my poor English. Thank you.
---- [[w:ja:mizusumashi]]