According to the “complementarity principle,” no!
The linguist François Grosjean found that bilingual children tend to acquire and use their languages differently: in different contexts, for different purposes, with different people.
They essentially develop complementary vocabularies, distributing the words they know across the languages they speak.
This is normal and to be expected!
A bilingual child who speaks Spanish at home and English at school might have the same “school words” as their monolingual English-speaking peers, but might have different “home words” that are available to them in English, because they speak Spanish at home.
Adults do this too! Sometimes people become more fluent in a language they learned as an adult.
For example, a Swedish scientist who learned scientific jargon in English might not have the vocabulary to describe their work in Swedish.
A Mandarin-speaker who moves to an English-speaking country as a child might only know familial words that a child might say to a parent in Mandarin, and have a professional and academic vocabulary only in English.
Bilingual children (and adults) aren’t behind if their languages aren’t perfectly balanced or if they don’t have the same vocabulary in both languages.
Instead, their vocabularies tend to be diversified, reflecting the words they need in different settings.
We learn the words that we need.
Source: “How You Say It: Why you talk the way you do and what it says about you” by Katherine D. Kinzler
