The 3 biggest of my children are from a first mariage and are now 20 (Mathias), 15 (Arthur) and 13 Sarah. Their father is French. Anna Frieda, now 2 years old, is born in France and her papa is German. I'm Danish !
The two boys were born in Denmark and their first language was Danish and secondly French; Mathias understood French but didn't talk the language when we lived in Denmark. Moving here was hard on him in the beginning; at that time I wanted to think that it was because of the language issue but today I'm sure that the cultureshift (and oh, kindergarden in DK compared to maternelle in France is like two different universes....!) was the reason for his failure to thrive in the beginning....:-(
We moved to France in 2000 and Sarah was born here. I was busy learning Frensh and talked mostly Frensh to her the first two years before I realized that she didn't understand Dansih very well...today I'm talking 90 % Danish to all the kids.
So, the children were and are exposed to Danish, French, English (because it's the language spoken at home with Anna's papa) and Anna is also exposed to German as her papa solely speaks German with her;
I asked the children what their attitude towards their different languages are and Mathias and Sarah looked at me as if they didn't really get it...Mathi said: yeah, it's sort of a part of me.... I think that means that they don't make any difference between the different languages....speaking and understanding more than one language is a fully integrated part of them and their personality.
OK, and to get there...? Rule number one: one person, one laguage ! Meaning that I only speak Danish with Anna, her papa only German, two of her siblings speaks French with her and the third one speaks Danish to her. Anna knows that I speak Frensh and English (creche, with friends in france and english with papa and other friends) but our communication base is Danish. I read and sing (not pretty...) in Danish and yes, at two she can tell the colors in German, Danish and French, her body-parts in German (because it's papa doing the bathing session), make the longest sentences in French (due to peppa pig in French and the creche) and the fruit and food part in Danish. She will speak a little later than her friends, same thing for Sarah and Arthur. At 3 they were more or less fluent, not before...Mathias was mainly exposed to Danish until her was 5 and spoke fluently at 18 month (but he was also a first child, 100 % time and attention on him !) So myth or not, I would like to know too, but there is not a lot of litterature/research out there.
Schoolwise they have been and are very different; Sarah gets only A+++++ (...), Arthur is dyslexic, but loves school, Mathi is very bright but can't sit still for more than an hour (only with the iPad...) I would go so far and say that the fact of knowing more languages is a benefit for understanding complex matters from math to geography ! And they are curious...
Generally I think that you play (as a mother or a father) a big role in bringing up multi langual children: you have to be conscious about talking your language 80-90 to your child, expose it to your culture (christmas traditions, food, classical books etc) and be proud of who you are !! And don't ever think that it's too hard for the child to learn more languages (common to hear from people (often only speaking one language themself):"oh that poor little brain"/"do you really think she's capable of understanding everything ?") IT'S ALL NATURAL AND PART OF THEM ! and it's a gift, their worlds are bigger, they will be understanding grown-ups, tolerant !
And a "friend" once said to me: don't make them think they are different bacause they speak more languages...But they are different, in a good way !!! Don't give up, resist negative comments from your surroundings, no the cow has different noises in different languages and the colors are not called the same, but they will get it, sooner or later;
Mixing up languages...some times but it's more on purpose and for the fun of it; like "rateau-ing" (one of them didn't know the work "rake") mixed French and English and we all understand ;-)
Only good things ?? Hmmm, the only "issue" is that my biggest son left home at 15 to go to school in Denmark; missing Denmark...he's now a chef, talented and working in an international environnement...my second son left last year to take his IB in Denmark, he's living at the school and I only see him every to-three months. He would like to continue studiyng, and asked me recently if it had to be either France or Denmark because he now got friends from Argentina and Singapore and he would like to go there too....(oh, no more easyjet-cheap-flights !) In July we will be moving to Germany, and Sarah will attend an international school too - and she would like to study in Canada...
The bad thing: expensive flight tickets and knowing that you brought up kids that will make their lives on another continent !! But it's worth it !! (and you see, I made another baby to compensate...joking !!)