it's worth worrying about a) the possible collapse of a common language and b)
the possible Quebec-like Mexification of Southern California
I don’t think the analogy between the situation in Southern California and in Quebec is a good fit. Quebec is a culturally and linguistically distinct region within a monolingual English nation. As such, the differences between it and the rest of the county are pronounced. This is nothing like the situation in the US. The geographic fluidity of immigrant populations, mirroring the mobility of the general US population, means there is no extreme cultural demarcation as in the case of Quebec. In the US, the Hispanic population is everywhere. When one thinks of Spanish speaking populations one thinks of New York or Miami, but even in North Carolina where I live there is a significant and rapidly growing Hispanic community. See also the recent LA Times article on the rebuilding of New Orleans. It will likely be rebuilt as a Hispanic city. Imagine how different the Canadian situation would be if the populations of, say, Vancouver and Calgary were each 30% or 40% French speaking. In short, even as Southern California becomes, increasing, a Spanish speaking region, it won’t be isolated from the rest of the country. The changes and challenges that the US faces and will continue to face as its Hispanic population grows certainly are significant. But it seems unlikely that they will be much like those in Canada. I’ll leave speculating on what the change will be for another time but expect new and exciting cultural formations, unlike ones we have seen before.
For more information on Latinos see this study http://www.brook.edu/es/urban/census/citygrowth.htm