Yes, it's very common. To go to Gilead School you must speak english (with the exceptions of the classes they have had in Mexico and Germany). If you want to move up in the foreign bethels, it is very helpfull to know english (Do you know any branch coordinators who don't speak english?) And I imagine the majority of Branch Committee members speak english. When a Zone overseer visits, from my experience he always speaks english. Almost everything is written in English and then translated to the other languages. Some of the translators aren't as good as others. When the translation for spanish was being done in Puerto Rico, there were all kinds of complaints. The meetings of the governing bodies are in English.
English is defenitely the language of Jehovah's Witnesses. If my memory servers me I beleive you will find in their literature a few comments showing that God choose the english language and America, as it was to be the most international language and where there was more religious freedom.