Will learning a second language interfere with development in the first language? 

The answer is no, definitely not. Generally, the effects from one language to another are positive. Research suggests that becoming bilingual has positive effects on language development, including on the first language. For example, when learning two languages, the child may become more sensitive and aware of language itself. There may be more sensitivity in communication and more awareness of the needs of listeners. Having two (or more) words for each object, idea or concept will expand rather than contract the mind.

The apparent assumption is that there is just enough room for one language inside the head. If one language is poured into that balloon, the mind will work maximally efficiently. If two languages are poured into the thinking quarters, the result will be two half-filled language balloons. Such under-filled language balloons in the bilingual will create an inefficient brain.

This ‘two balloon' concept is wrong. First, there is more than enough room inside the thinking quarters for two or more languages. It seems impossible to set limits on the amount of learning, understanding and knowledge that a person can hold within their thinking quarters. As in the example of a child taught mathematical multiplication and division in one language, those concepts do not have to be retaught in the second language. They immediately transfer as an idea and an understanding into another language (so long as the child has the vocabulary to reproduce it in that second language). Thus, the two language balloons merge inside the thinking quarters. There is a common thinking area that can be serviced and supplied by both languages.

So, rather than a second language interfering with the development of the first language, it is more likely to provide thinking advantages, social and cultural advantages, even economic advantages in the long term.