12th Century? Pretty difficult. The Old English period goes on for a little bit after 1066, but then you start to see Middle English-isms creeping into the texts. (see the Peterborough Chronicle vs The Anglo-Saxon Chronicle)

Old English was very similar to Old Saxon, which was spoken in Northern Germany. It's so similar that we have an Old English version of Genesis (not the book itself, but a poem based on the book) that has gaps filled in with the Old Saxon version. Technically, the scribe 'translated' it, but it really didn't need much translating.

Old Frisian was also fairly similar to Old English. Old Low Franconian (a fancy name for Old Dutch) was less so.

I could easily see an Old English speaker, before 1066, getting by in conversation with Frisian and Saxon speakers, maybe with some effort. Once we start to get the beginnings of Middle English, the languages are too different. As a reader, I have a hard time with some of the really late Old English texts, while I find Old Saxon comes off as pretty similar to the Old English that I'm comfortable with, but with odd spelling.

South of the regions where Old Saxon and Old Frisian were spoken, there wouldn't be much mutual intelligibility, even before the 12th century. Now if you go back to the 5th and 6th centuries, maybe. My first old Germanic language was Gothic, and reconstructed Proto-English words are often almost identical to their Gothic forms. I can usually guess what they're going to be without even taking sound shifts into consideration. Western Germanic speakers would have been even more similar to each other.