In general, the return on investment for a native speaker of English learning any language as a second language is lower than the return for the same person learning a lot of mathematics. People who don't know English usually find better gains by learning English, but they still have to learn some other career skill as well. The least expensive and most effective way to learn a language is to learn a bit of it before foreign residence (so that you'll live in fear of not knowing the language, and study hard) following that by foreign residence and continued part-time focused language study while working and participating in the culture in the new country. And that's just how a lot of second-language speakers of English reach very high levels of proficiency in English.