Language fallback to Spanish (es)
Even if those numbers where made up numbers (I don't mean they are). I think is more likely for a Basque/French speaking user to be fluent in English than in Spanish, so I agree that Spanish should not be the fallback language.
I'm sorry but it's extremely unlikely that more than 85 % of the inhabitants knows English. The average for Spain is 24 % and for France 39 %.
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.