I think it depends of the language and the type of fandom. For instance, the Japanese fandoms are pretty much hermetic and self-sufficient; while you CAN see Japanese-speaking people hanging around in English-speaking fandoms, especially in LJ, that's not the norm. Most Japanese fans won't bother to learn another language, not even when they're Western fandoms -if it's a Japanese fandom like manganime and videogames, then there's not even a question. This is why there are so many fansites in Japanese only, with fanworks made by and for Japanese fandom.
Now speaking for Spanish fandoms, it really does depend of the kind of fandom we're talking about. Anime/manga fandoms prosper and grow to be extremely popular only if the series is broadcasted on TV (in several countries). You rarely see a full-grown fandom in Spanish for a series you can only watch via fansubs or for a manga -be it licensed where you live or not. Of course there are exceptions: there were solid fanbases for stuff like Naruto and Bleach from before, but again it's normally for long shounen or massive mangaka like CLAMP. With the tiny fandoms, you might see people commenting on it and even discussions, but not... like, too many fanworks (and p0rn) in Spanish. This might be why those who can speak even a bit of English might jump to those way larger fandoms in English to get their fix or write around, but people who can't speak it will prefer to stay in their other big Spanish fandoms instead of checking out small(er) fandoms in English or creating them in Spanish.
With videogames I have no idea how Spanish fandoms work. All I see around are generic gaming forums/blogs full of fanboys not unlike in English. I bet there's shipping and slash and shipping wars, but it's not easy to find. Keep in mind that in Latin America we get games untranslated and not everyone can get a console, so most gaming fangirls know more than enough English to join the English-speaking fandoms. Exception is Spain because Europe is weird like that. I have no idea what non-English Spaniards fangirls do about.
With books... there's a Spanish Harry Potter fandom, and it's quite large. I doubt other book series might have this kind of full-fleshed fandom. And yet, I know that a lot of Spanish-speaking fans still check out the English one if what they like is too obscure or they're fluid (I know Laurus does go to English corners).
With real life fandoms like TV shows and movies? Well, the thing is I don't hang around these fans a lot. I know that in LJ there are a lot of TV-addicts who keep themselves up to date with the shows via downloads and cable TV, and they friend each other and have their little groups, and ship characters around. But for large fandoms (House, Who, SPN, anything by Josh Whedon...) I have no clue if they have their own communities, or if they do fanworks (and whether they're in Spanish or English), or what non-English-speaking people do besides interacting with their flist -those fans CAN be almost as updated as the other fans, thanks to soft subs and downloaded episodes. I can theorize, however, that smaller shows are not going to have a solid Spanish fandom, and those who can will flock to the already tiny English fandom not unlike anime fans. But again, the few Spanish friends I have into TV and movies are fluid in English and just join directly the English fandoms, so I really don't know enough.
picword: Fantasia / I brake for grape