not sure what language Word is written in, but Windows is written in C if im not mistaken. Alot of people use simpler languages like Python for writing simple apps.
You could write a word processor in pretty much any language under the sun - what languages do you know, if any? That might be a start. Word processors are so common these days it's more of an academic exercise to write one than anything else, so pick a language you want to learn / gain experience in and go for it.