I would say, Bifur for sure can read Druid scrolls. It would be weird if he couldn't use a major ability of his class! So Bilfur definitely can read well enough, to know that this is not a Druid scroll.
Whether or not Bilfur can read and write English, I will leave to you as a role-playing decision. There is nothing in the rules that Int 6 characters must be illiterate. I personally think Int 6 is like a C student level of intelligence: You are below average in your class, sometimes it takes you a second or third try, but you get passing grades.
You've mentioned a few times that Bilfur is a feral child of nature, so if you want to say, therefore he can't read English, I think that is a fun role-playing decision. Either way, let's say he knows enough of the wild Druid-runes that he can read a spell-scroll or communicate with other druids.
Now that Bilfur knows the scroll is not Druidic, if he wants to hand it off to a higher-Int party member, that might be a good compromise. When the party reaches the surface of course!