When to start encouraging my bilingual child not to mix languages?

Question

I am a native English speaker and my wife is a native Spanish speaker and we both speak the others' language fluently. We live in a predominently English-speaking country. We have been raising our two-year-old son to be bilingual where I speak mostly English to him and she speaks mostly Spanish with him. We do however occasionally use the other's language with him and each other but we never mix languages in the same sentence or paragraph.

When reading counting books with my son, he will often say something like this:

 uno, dos, tres, four, cinco, seis, seven, eight, nine, ten

First, if we start counting in one language should I correct him when he uses a number from another language?

Second, at what age should we start introducing the concept of two languages and say "Let's count in English" or "Let's count in Spanish"?

He knows some words in only one language and other words in both. For example, he uses airplane/avión interchangeably. He is not stringing too many words together to in sentences yet but I suspect that when he does, he will also start mixing those. What is the best way to address mixing?

Answer 1

Short answer:

Now.

Long answer:

The common approach to teaching bilingual children is "one person - one language". Little kids are confused by contextual language switching (e.g. we speak one language in private and another in public). But they easily associate language to person.

Some research claims that ability to express oneself completely in one language, and frequent language switching (while talking to different people) is what helps a child to maintain minority language skills later on. Supposedly, it is important how easily the child switches between the languages, not how well the child learns the language in the early years. Hence parents should motivate the child to speak one language at a time and switch frequently, with the goal of making a child self-sufficient in both languages.

From personal experience... When a child enters school (4-6 years old) and most of its communication becomes in majority language, the child rapidly looses minority language. The ability to learn a lot is compensated by the ability to forget anything that is not used every day. If the child had difficulty expressing complete ideas in minority language before school, he/she may fully switch to majority language in elementary school.

My son learned basic counting (one to twenty) in both languages through respective parents. Advanced counting (up to a hundred, back, forth, odd, even) was practiced at the age of 3.5 - 4. At this age I asked him to do some tasks in one language, some in the other.

Answer 2

We are a monolingual family living in a foreign country. Our daughter, now almost four years old, clearly understands the concept of foreign languages and can count in our native language and two foreign ones. Her other use of the foreign languages is much more limited.

As such, a three-and-a-half year old girl seems to understand the concept of different languages, given explicit discussions of this (for example, "people speak Danish at the kindergarten", "now we are singing the ABC song in English", and so on). As far as I know, boys tend to develop more slowly.

I usually correct my daughter when she mixes languages, or ask things like "What is lyserød in English?" (which are usually quite challenging questions at this point). I can't speak to the effectiveness of this.

I also can't comment on mixing languages, as we have one strong main language and the others are more like spices, or necessities for communicating with the outside world.