It should be possible to e.g. query the French wiki through the English wiki API, like
This is needed for scripts using XHR, which can't go off-domain. An implementation should preferably add a table mapping interwiki prefixes to api.php URLs (maybe use the existing interwiki table for this?), then kind of forward the request by acting as a proxy (send an HTTP request to fr.wikipedia.org and spitting the output back out to the user). 302 and friends probably (?) won't work here (XHR's gotta be smarter than that).