I often write eMails in german with english words... (or I write a german or a english email... )
Every time I need to switch between these two dictionaries. I know how to do but many users doesn´t know or it need to much time... A good idea would be to use two dictionaries. A primary and a secondary. So when I write a english word in my german email it will be not red underlined because it will be found in the secondary dictionarie.