Eh, maybe not. You only have a limited number of neurons, and they can only form a limited number of synapses. 70-80% of your neurons aren't even related to learning or remembering, they'll all in your cerebellum. Luckily, your brain automatically forgets unimportant memories so that new memories can take their place.

I think you should develop passive skill on your language, just like what I do with japanese right now. C'mon, french is not that hard. I understand it much faster than I do with japanese (probably because I have learned german as well, which makes me easier to understand romance language in future).

hmm... are you sure?

The average human only uses like 12% of their brain's capability, or maybe it was 2%.

Once something goes into our brain, it stays there, and when we say we have completely forgotten something, it's not because it was erased from the brain, it's just that you never think of it, that's why you couldn't recall that certain memory.

back to topic, my native language is mandarin, but I've lived in the US for almost 9 years.. I still do my multiplication's table in mandarin xD, but instead of thinking in mandarin, I think in English now for the most part. Like when I first started to learn English, I'd have to translate every word in my head to chinese first in order to understand. Now it's the other way around.. for the most part.

No. Normal humans use the enitre brain. Otherwise we would be able to remove a large amount of our brain without issue.

WHICH CLEARLY ISN'T THE CASE.

I can speak Japanese, Spanish and English.
I often start speaking Japanese in Spanish class.
So I'm really not too sure.