Will my child progress more slowly in his/her native language because s/he is also working on another language?

Yes and No. Yes in that s/he may be acquiring vocabulary and expressions in the non-home language because of encountering them at school, however lexical items are the easiest elements of a language to acquire and as soon as a child is exposed to the item and needs the item, s/he will acquire it. No in that s/he is still acquiring the deep structures and concepts of the home langauge which will emerge later.

Much depends on the individual child, and the circumstances at school and at home. Research has shown that, where a child has reached a certain level of oral competence in his or her primary language, a second oral language poses no great obstacle for continued progress in the first; there is generally little or no confusion for the child and s/he will progress in both languages, and in both oral and written forms without detriment to either language.

However, progress in both languages may be slower for some children than if those children were studying only one language, especially if comparisons are made on a vocabulary level. Nonetheless, the benefits gained through acquiring more than one language considerably outweigh any possible disadvantages and, after a certain number of years, especially after the languages have been fully integrated into the child's learning experience, progress in the native language will proceed as quickly as for a child learning only his/her native language.