Is the ability to speak a language strictly environmental?

Our native language (mother tongue) is learned through exposure to the language (parents and others). The ability to learn a native or other language can be enhanced or hindered, like most other skills, by in-born (genetic) factors. Referring to your example, the Japanese child being in Japanese speaking surroundings will most likely learn the Japanese language faster than it would have learned it in Kenya. Also remember age plays a vital role. An infant learns by hearing and seeing whereas an adult, or near adult, most of the times will have to learn a new language by following some prescribed teaching method Being exclusively in foreign language surroundings will expedite this process (forced to listen, speak and read). This is a social process.