I write in Finnish, so in most cases if I receive an email in Finnish I try to reply in Finnish. I will reply in English if I'm busy or if there is something urgent, because writing in Finnish takes more time. Also I have a person who helps me with emails in Finnish sometimes.
As for speaking, I can speak in daily situations, but I don't feel confident to discuss work in Finnish yet. So, when I meet with clients/talk on the phone I inform beforehand that it would be great if we used English. But please be sure I understand everything you write/almost everything you tell me in Finnish😉