You are probably an outlier, in that majority of ip requests coming from spain are going to want spanish. it makes sense to render default in spanish. I am not sure what headers your ipad is sending, but i doubt it sends what your language preference is. (i could be wrong)

also, as previous sites don't add the chosen language in the click request, i don't see how being on hn previously is relevant.

the flip side to this is - if this is actually a big problem (incorrect renderings of language) maybe some dnn could be applied for a new start up idea! haha