About [[MediaWiki:Vector-collapsiblenav-preference/en]]

I rather would leave it as is, since "sidebar" is a technical term, that we know, but the average user will likely not be able to recognize, or tell it from the other "bar"s. --Purodha Blissenbach

And how is the dull user supposed to now what is navigation bar when developers including my cannot work it out?

I agree with Nike, the message must be changed. If it's the sidebar, then call it sidebar. The meaning is obvious, and even easily translatable (there isn't any other bar on the side of something).