I don't really think is the case. I haven't noticed what you say, but I think such phenomenon could be explained by saying that when people become more fluent/proficient in Spanish they ask a "higher level" kind of questions, which other users might find more interesting and thus get more upvotes. Since these users are fluent/proficient in Spanish, they try to post their questions in Spanish (to practice a little bit further).
Also, native Spanish speakers would not post, for obvious reasons, easily answerable questions. The kind of questions a native Spanish speaker would ask here should at least spark the curiosity of people who are part of a site about the Spanish language (meaning, interested about these sort of things).
So my guess is that the most interesting questions (the ones that would get more upvotes) get to be written in Spanish, but not that just for being in Spanish a question (or answer) would get more upvotes.
As a native Spanish speaker myself, I have to acknowledge that I have learned a lot about the language I thought I mastered just for being native (I neglected learning properly the reasons why things are a certain way when I was in highschool, so answering some questions has forced me to try to bring myself to a level where I'm able to explain them to others). So I admit I would be more prone to reward with an upvote a question that would make me learn/relearn something interesting about the language (maybe something about grammar? regional differences? something that would help me improve the way I speak or write?) than just a "translation for this or that".
Also, take into account that this stack is in beta. It receives far less questions than other stacks, and maybe users visit less often and are less prone to vote or contribute. Older (the oldest) questions from the private beta have far more upvotes than the questions being asked right now.
This stack may also work in a different way to others, like StackOverflow, where a dummy question about a mainstream programming language would get more upvotes, just because more people would trip in that stone and find the question/answer helpful and valuable, while a really high level question gets less votes, since less people would experience that same problem.
Thus said, I have to tell you that I vote almost every question where I place an answer (I wouldn't be answering it if I didn't consider it interesting or worth checking) and I upvote answers to support good answers or what I consider to be the right answer to stand out. I just don't upvote everything and I'm also critic with poorly written answers (or questions). I don't really care in which language they are written, as long as they are not written in the shoddy/lazy style that is pervasive in the Internet these days. I agree that some questions and answers seem not to have the proper credit they deserve, but again, the site is in beta version, and each user may have their own criteria to grant an upvote.