Some of my engineer colleagues are very hard to understand on the phone because of their language. It's hard to imagine them getting anywhere as a lawyer, for example.
Thats not correct theory. Visas (h1b) are only granted where there is perceived shortage like computer programmers ect.
An indian woman cannot come to "choose" to be an english teacher in usa.
Some Indians speak English fluently, but in a way that most British people find incomprehensible. In contrast, and somewhat ironically, a German speaking bad English is often easier to understand than a random British person (as opposed to a British person from one's own social class and locality).
The level of English knowledge you need to study maths or engineering at a British university is obviously very much lower than the level required for studying law or working as a lawyer. Some otherwise intelligent people are bad at languages and will realistically never achieve knowledge of another language to a level sufficient to successfully work as a lawyer. In fact, I have been reliably informed that some Germans give up on physics as a career because they realise that they will never manage to learn English well enough to succeed in academia, which is a bit sad, really.