A researcher, Bente Ailin Svendsen, in linguistics at the university in Oslo has concluded that bilingualism is an advantage (Norw. article). It doesn't lead to confusion; it leads to greater flexibility. The article says about her findings (quoted text my translation):
Bilinguals react faster to stimuli than do monolinguals, and the brain seems to stay in better shape. Studies from the US and Norway show that students with long-term bilingual education do best at school.
Ah. I do react fast to stimuli, to the point that I've been wondering if I have ADHD. I did do well in school, too, but I was a smart kid before I learned Norwegian - which is probably why I learned enough Norwegian in three months to move up a class. I was supposed to be in third grade (today's fourth), but the Norwegian school started me in second so I could learn the language without being "bothered" by learning new stuff as well. By Christmas I was fully ready to move to my proper grade (I was also motivated by boredom, and the fact that one of the second grade girls was really mean to me).
"They also seem to be more empathic, perhaps because they can better see a situation from more than one perspective. People who know several languages aren't more intelligent, but their brains are more flexible," says Svendsen. Multilingualism also seems to prevent Alzheimer's.
Grandma told me a number of times that you never learn more about your own country than when you leave it. Living abroad makes an American more keenly aware of what is typically or uniquely American. Language is part of it. Language is culture and history and habit and custom. Learning a language teaches you about the people who use that language. With language you learn verbal habits, local sayings - and humor. Without understanding the local humor, you will never understand the people and you will never feel at home (is my experience). Being able to share a laugh, being able to be the one to make the others laugh, depends on language. I can remember feeling downright handicapped before I learned to quip as fast in Norwegian as I did in English. It's something about being able to participate.
"Researchers used to think that children would be confused by multiple languages and end up not knowing any of them well. Now it is a common belief in linguistics that knowing several languages well aids a child's cognitive and verbal development," Svendsen says.
I was never confused as a child. (No, really.) I was always clear on English being English and Norwegian being Norwegian. I do, however, tell people that my brain doesn't always bother keeping the two apart, but will just assume it has twice the vocabulary. I once claimed that my underarms had a funny rash and couldn't fathom why my fellow English speakers started going on about switching deodorants. It finally dawned on me that I was using the Norwegian word for lower arm, which is what I meant, and not armpit. That happens on occasion, especially when I'm tired. My brain picks the first and best word it can find, regardless of origin. Even my grandma would do that. She couldn't find the English word, but the Norwegian one was right there, ready to be used in an English conversation - or vice-versa.
The ultimate in linguistic confusion was Grandpa's. He'd learned English as an adult, and spoke it fluently, if heavily accented. What got us was his habit of answering Norwegians in English, and Americans in Norwegian, especially on the phone. Overseas calls from Mom, who speaks only English, would consistently be answered in Norwegian by Grandpa. Norwegian relatives on the phone learned English when talking to Grandpa. Our bilingualism got the whole family flexible. :-)