do backlinks from foreigh websites still carry value?

For example, links from Spanish websites to an English website, or vice-versa

wouldn't it be obviously un-natural to have a Spanish page with some English anchor text backlinks?

or even an entire English page on an otherwise all Spanish website?

It'd be highly unnatural but they still transfer link juice nonetheless and there's proof of that everywhere. Just take a look at the BST section and you'll find services selling russian backlinks to English webmasters and they are reporting good results.