When words come quickly and my fingers barely can keep up with the stream of thoughts I usually don’t stop to look up a word in a dictionary. My English vocabulary isn’t too big unfortunately so I sometimes use words in three different languages. My own, that of the country I live in and English. In this way I keep the flow when freewriting.
When editing I translate the foreign words into English and this helps me to build a vocabulary of my own.
Sometimes these non-English words express something in a certain way that can be hard to translate and that keeps my writing personal.