I am writing a cover letter in English to The Netherlands. I don't know whether to use English when writing the address or Dutch (in the letter, not on the envelope). All the terms are translatable to English (department code, attention, mailbox, etc.). Using English or Dutch seem both out of order. I was thinking to use English for the terms, such as department code and attention, but Dutch for the actual address (mailbox/postbus).
What is common when sending a letter abroad in English writing?