Hallo! It’s from a thread in the Russian forum. I’ve always thought that it should be “his mother tongue”. I know that after “everybody”, “everything”, “no one” the verb should be in singular form and pronouns must be singular as well. Everybody who has not purchased a ticket should be in this line. (TOEFL preparation guide by Michael A. Pyle, M.A., Mary Ellen Muñoz, M.A.) True, that in that book it’s said “in formal written English”, still I’m interested in your opinions.