I think that you should go for it and speak Greek even though you may be a little self conscious about it.
Afterall there are alot of us here that do quite a job on the English language
I had 3 Cuban roommates in college and they often took me home for weekends. Their families ONLY spoke Spanish and I was taking Spanish in college and would practice it with them. They actually had a thing against American girls and were going to pull their daughters out of college once they learned that they had an Amereican roommmate
That is, until I went home and tried my Spanish out on them and they learned to adopt me as their American daughter.
I would call them Mami & Papi just like their daughters did and to this day consider them as so.
So give your BF's family the same chance to get to know you!!! I guarantee that it will be a great experience ~ Goody