Which parts of Wolverine's brain did the bullet hit?
The script doesn't describe exactly what parts of the brain is hit:
Stryker fires again.
The adamantium bullet rips through Logan's forehead with an unholy sound.
Logan falls to the ground.
Stryker stands above him and fires another bullet into Logan's head.
We already know from the other X-men movies that Wolverine has lost part of his memory so it stands to reason it has hit the parietal lobe, but seeing as most people don't know much about how the brain works (or know that "We don't really know how the brain works" as everyone says in movies and TV), I don't think David Benioff & Skip Woods were that concerned with explaining the exact trajectory and damage of the bullets.
That said, the brain is concave and there is a small gap between the left and right hemispheres, so the bullets could have ricocheted off the top of the inside of the skull down towards the parietal lobe.
How come he knows how to speak English, and how come he doesn't have his memories?
Sufferers of Retrograde Amnesia are more likely to lose recent memories that are closer to the trauma than their older memories.
It may be that the bullet isn't even the primary cause of the amnesia, for example in the Comic Origin he loses his memory due to shock:
James is still in shock and appears to have no recollection of what happened. This loss of memory is due to Wolverine's/James' healing factor which, in effect, "healed" (by putting up a mental block) the mentally devastating traumas of witnessing his father's death, the confusion of mother's anger towards him and the pain and surprise caused by the sudden manifestation of his claws.
With this in mind, it could be the fact that he just saw Kayla die in front of him, had liquid metal injected into him and had his brother turn against him that was the real cause, and the bullets were just the finishing touches.
Regardless of the cause, TVTropes calls this particular plot contrivance Laser-Guided Amnesia:
With surgical precision, amnesia strips you of all information pertaining to personal identity, leaving just about everything else intact. TV Amnesia is a disorder where you forget where you put your keys, but you do not forget what a key is or what it's for. You will forget where you went to school, but not any of the things you learned in school. As a result, the character will retain all of their skills — though they may not know they have them at first.