Our website has many articles in English, and perhaps 20% have Spanish versions. These are very carefully translated, fixed texts (not translated on the fly). This is California so providing Spanish version where we can is important, but speakers of one language can usually understand a few key words in the other.
Is there a consensus on the best or most effective placement for these? Is there evidence whether users prefer all articles on the same topic listed together, versus all articles in the same language listed together?

Edited to add: This is exactly what we do on the content pages: if there's a Spanish version each has an obvious button to get to the other-language version.
My question is what to do with, for instance, a listing on another page of the six publications about Ants, 3 of which have Spanish versions. List Eng. & Sp. separately in the list? All Sp. in separate list?
– Chris Aug 12 '14 at 18:37My question is what to do with, for instance, a listing on another page of the six publications about Ants, 3 of which have Spanish versions. List English & Spanish versions of each article next to each other? Or all English articles in one list, Spanish in a separate list?
– Chris Aug 12 '14 at 18:45