I was always taught that you should first introduce the language your child will use in school and society. When a child knows the primary language well, then introduce the second language. This helps avoid confusion of blending both languages. For example, when my daughter could count in English, I taught her how to count in Norwegian. She was 2 at the time and this seemed to work very well. In fact, she finds learning languages easy because she was introduced to more than one language before the age of 6.
Here are some interesting ideas about bilingual language. In fact, I find this quote by Dr. Silvana Montanaro, MD very interesting:
“If we could have two, three, four or five different persons speaking different languages around the child, she could easily absorb all of them without any particular effort, provided that each person speaks to her ALWAYS AND ONLY in their language. But this is possible only in the first years of life.”-
I wonder if anyone has ever done this and documented it?