Language fallback to Spanish (es)
This is not about inhabitants, but about users. And basque language users don't use Wikimedia projects because they don't know spanish/french, but because they want to use it in Basque. So if something is not in Basque language they will [politically, socially, technically] understand having it in English better than reading it in Spanish/French. If they wanted to have sentences in Spanish they would read in Spanish in first order, as most of them know the language.