I am not a native English speaker but I know far more English than scientific names - hell, I know more English than Czech names, because I cannot be bothered with Czech names for birds outside WP and all the books I have are in English anyway. The scientific names are often hard to remember as they don't mean anything in any language I would know. I know that hardcore birders sometimes know scientific names and I have used this on some occasions, also in Poland (because Czech and Polish names of birds are super different, despite the normal words being quite similar), but I would say that most people you meet in the field do not really know many scientific names.