The Arabic wikimedia projects has underscore(_) instaed of space in there names, according to https://www.mediawiki.org/wiki/Manual:$wgSitename their are not any problem with spaces. Using underscore(_) in place of space make text ugly and like hardcoded, because many of mediawiki messeges uses {{SITENAME}}.
there is one project uses space not underscore, it is arwikiversity (ويكي الجامعة) without any errors.
These projects are:
arwikisource = ويكي_مصدر -> ويكي مصدر
arwikibooks = ويكي_الكتب -> ويكي الكتب
arwikiquote = ويكي_الاقتباس -> ويكي الاقتباس
arwikinews = ويكي_الأخبار -> ويكي الأخبار
I request replacing the underscore(_) with spaces in their names