On beta features tab in your user preferences http://de.wikipedia.org/wiki/Spezial:Einstellungen?useskin=modern&uselang=de#mw-prefsection-betafeatures visualeditor shows a red requirement warning if the current skin is not supported. But this warning uses not available system messages:

This feature only works with the following skins:

Vector
<skinname-apex>
MonoBook
<skinname-minerva>

The problem on wikimedia wikis is that both skins, apex and minerva, are not installed and so the system messages skinname-apex and skinname-minerva are missing.

I think there are two options how to fix this bug:

1. add these two messages to localisation files used by wmf
2. test if a skin is available before adding it to the skin list of the warning message

Version: master
Severity: normal
URL: http://de.wikipedia.org/wiki/Spezial:Einstellungen?useskin=modern#mw-input-wpvisualeditor-enable