- The slogan "Wikipedia is a non-profit" sounds weird to me.
Technically, Wikipedia is not "'a" non-profit (which in my English as a second language view of the world, means organisations) . However, Wikipedia is non-profi, ie. the project is not meant for profit.
Maybe that doesn't work in English, I don't know. I suppose that the site notice is going to be translated, so maybe make sure that the translation makes sense rather than reflects the English text. Also, in many languages (well, at least in French) non-profit is a huge set of words. We might want to "localize" the slogans rather than just "translate them".
The same applies to English. "non-profit" is short for "non-profit organisation", which Wikipedia isn't. "Wikipedia is not for profit" might be better.