LOVE this subject - especially the opportunity to rant on about my own experience with no need to back it up scientifically....
But, a (semi) scientific point first - there is a great book called 'The Language Instinct' by Stephen Pinker that I would recommend if you are interested in this area. According to him the research shows that humans acquire 'grammar' around the age of two or three, and that they acquire grammar regardless of the ungrammaticality (!) of the language spoken to them. To support this he talks of the difference between 'pidgin' - a means of communicating for adults with different native tongues, that has no consistent grammar - and 'patois' where words are adopted from different native tongues by children who meld the whole with a consistent grammar to create a new language. He also compares people who have been locked away from human contact (usually by psychotic parents) until different ages and looks at the langauge they end up learning. Those who learn late apparently never really 'get' grammar, which severely limits their ability to communicate.
(I may have all that wrong, but that's what I remember from it in this context.)
As for the language you speak affecting your character, I 100% believe this. The person I am when speaking Japanese is quite quite different from the person I am when speaking English.
I've also always thought it interesting how the same sentiment may be very easy to say in one language but hard in another:
'I miss you.' 'Tu me manques.' (literally 'you are lacking for me.') Note which person the emphasis is on and you can see why we rough egotistical Anglo-Saxons have taken over so much of the world.
NB It's much much harder in Japanese and I can't even think of a way of saying it that I wouldn't have to check with a native speaker. Damn! NOw I'll have to go and ask...
