If I was to travel back to 12th century England, would we be able to easily understand each other's use of the English Language?

No, you would not be able to understand the language nor would you be understood. You would be hearing Middle English, and early Middle English at that (as the transition between Old English and Middle English happens around the 11th century). You might hear a word here or there, but the majority of the language would be incomprehensible.

For an example of a 12th Century work in Middle English, you would look at The Ormulum, a biblical interpretation by Orm, an English monk.

http://quod.lib.umich.edu/c/cme/AFW5744.0001.001/1:7.2?rgn=div2;view=fulltext

To hear how it sounds, this is one of the few audio links I could find of The Ormulum:

https://www.youtube.com/watch?v=cyNEhEJOr8w

This presents a good image of English pronunciation following a major phonetic shift post-Norman conquest of England

For an example of how the English language developed from the 12th Century, take a look at Chaucer's The Canterbury Tales, written in the late 14th century:

http://quod.lib.umich.edu/c/cme/CT/1:1.1?rgn=div2;view=fulltext

This represents two hundred years more development toward Early Modern English (Shakespeare's plays) than the 12th century would. It is a bit more comprehensible than The Ormulum, and we can begin to hear (and see) the shift toward Early Modern English.

Here is a reading of the prologue of Chaucer's The Canterbury Tales for comparison:

https://www.youtube.com/watch?v=GihrWuysnrc

Moreover, if you want to progress into Early Modern English (Shakespearean English), you may be surprised to find that the pronunciation is not what you expect as well. As a bonus, here's a video from a Shakespearean scholar explaining the difference between Shakespeare's pronunciation and modern pronunciation:

https://www.youtube.com/watch?v=gPlpphT7n9s

As you can hear, the original Shakespearean pronunciation is a direct phonetic descendant of what we hear in The Canterbury Tales.