Usability studies on language switches and multi-lingual content websites

Does anyone have actual usability study / testing research around the most effective solutions for language switches, or even dealing with linguistically fragmented content on an information website?

I'm working on an information website that's mostly in English, but has some pages in Spanish. I'd like to examine any established secondary research that shows what has been tested with regard to efficacy of switching languages, or perceived gaps in language content parity.

ux.stackexchange has plenty of opinions and commentary on the subject, but I've struggled to find actual usability testing results on the subject.