I guess that is because many of the teachers can't speak well enough themselves! It all depends what situation you are going to use the language in. If it is (say) in a business context, where they are writing emails, then just having good written skills is fine. Obviously if you want to talk to people on a 1:1 basis, you need to be able to speak as well. How well depends again on the situation. If you were negotiating a business deal, then you need to be very good at spoken language! You need an 's' on teachers. And 'professionally' usually means 'good enough to be paid to do it' -e.g. as an interpreter. So maybe that isn't the word you meant?