I also think that it depends on the country and its conceptions. Because for example here in Spain you can find
english as a foreign language, which is basically a non-matternal language that you're learning on porpouse rather than
naturally, and
english as a second language means, as far as I'm concerned, that you have alredy chosen another language to learn after your matternal language. For example, here in Spain we mainly study English as a foreign language and later we (may) start French as a second language
(though we're horribly bad at both of them

)