Info. required on a Mastiff (Mastin Espanol).......?

We are living in Galicia, Spain and about a week ago an abandoned (the Spanish are good at abandoning amimals and we have already adopted a spanish cat and dog) Mastin dog appeared near our house. These are native spainsh dogs and I don't know a lot about them. I am presently feeding it and obviously it is quite happy to stay around! It is very nervous of strangers although with myself and two children it is very friendly, giving the paw etc. buit with my husband still v. scared and backs away. Is there anything I should know or be aware of???