I also have long felt that anyone who has the ability to read and write at a high-school level in their primary language also has the mental capacity to learn to do math at a high school level. As a pedagogical challenge, I think the main stumbling block for most people with math is not the complexity of it but rather the dryness of how its taught. The rules of language are at least as complex, but many more people learn language at a high-school level than math. There are lots of reasons for that, most obviously language is used more in people's every day lives.