My Q&A community operates in English. Posts sometimes quote sources in another language, which we allow so long as the essential information is summarized in (or translated to) English. We have a sizable minority (guessing 10-20%) who are fluent in the other language. We've been asked to consider allowing posts in the other language without requiring English accommodation.
How does a site being bilingual (or multilingual) affect user engagement? On the one hand, you gain users for whom English isn't comfortable; on the other hand, very few users can now read (or vote on or review or moderate) all the content on the site any more. Do users not fluent in both languages see that as alienating, or do they treat posts in other languages as "just another topic I don't follow", like questions in a tag a user isn't interested in?
I know that Stack Overflow created separate sites for other languages instead of mingling questions in other languages on the same topics into the English-language site. I don't know the extent to which they studied user engagement in making that decision. SO is huge, so there might have also been technical issues. We are not huge; there are no unsolvable technical barriers to supporting two languages, though there would be policy impact. My question is about how such a change would affect our users.
Are there any case studies or research on this?