If someone outside of US wants to be a professor in US, do you think he should focus on studying English, or it is ok to study his interest language too or its's a negative impact on learning English (English level is fine, but not professor level)?

In the US, it is VERY different than in Europe and most other places.  Usually,  most people can speak 3-4 languages in Western Europe and South America.  In other advanced places in the world,  most  speak 2-3 languages, 

However, IN THE USA, to be a professor,  you need to speak English VERY WELL because most here speak only English as the main language.  It's different here. We don't have countries right next to each other here like Europe does to where we need to know other languages. . So yes, practice English,  and even then it's still not the same as LIVING among English speaking people. You would need a year of IMMERSION to really grasp the concept of English, meaning you would need a year of living in the USA to really grasp all the strange little sayings and slang. Keep studying your interest language,  but you need that English to be very strong.