I love San Antonio. I live centrally to each of Texas' major cities, and San Antonio is probably my favorite (Ft. Worth the other one I love).
I think learning Spanish would probably only depend on your career and how much you have to work with those who speak Spanish. After a while you do pick up any of the words and phrases you might need-- like "how many tamales can you make me this week because they are so damn good I can't live without them." Yes...almost everyone I know knows an old mexican woman who makes as many tamales as she can about once a month or so and sells them to whoever will buy. Mmmmm!!!!!
Downtown and Riverwalk are fun...but they get old. The great thing about SA is that it has the benefits of a major city-- like pro sports, great restaurants, concerts, etc. but lacks a lot of the congestion and stuff you get in larger cities. In addition, there is so much do to nearby-- Sea World, Johnson Space Center, Schlitterbahn (the biggest and best water park in the country)....and all sorts of stuff. The weather makes it so that you don't have time to do all the things you want to do. Riding is great, riders around TX are cool people, roads are excellent, etc.
And yes, cost of living is extremely low. You could find a wonderful starter home for $150k. It's beautiful and hilly.
Should you come for a visit, I'd be happy to venture to SA for the wekend and show you around, introduce you to some people who know the area even better than I...etc.
Oh...last thing...you asked about the people...
The people there, IMO are a bit more cosmopolitan that in other Texas cities. That's not to say they ARE cosmopolotan, but that they aren't as hickish. People there tend to have more respect for people of other cultures and races (I would guess due to the high pop of mexicans). I find them to not be as snooty as people in Austin, not as spolied as those in Dallas, and not as white-bread as those from really small farm towns. Very nice down to earth people. I think coming from Chicago, San Antonio would be the best city in TX to transition to. I think the biggest shock you will find in the physical size of the cities here-- they are quite small in the downtown area, and then everything is very suburban and spread out.