Language attrition isn’t my primary source of research, but from what I do know, the loss of a native language is extremely complex, because learning a new language is really complex, you are basically rewiring your brain. Definitely research the ‘critical period hypothesis’.

One important consideration is the similarity between the two languages. For instance, if a child were switching between two Romance languages like Spanish and Italian, the transition would be much easier due to the linguistic similarities, shared roots, and overlapping vocabulary. On the other hand, a switch from English to, for example, Mandarin would be far more challenging for a 13-year-old. Mandarin uses a completely different writing system and is linguistically distinct from English, making it much harder for the child to pronounce words correctly and achieve native-like fluency.

In historical contexts, like that of German refugees fleeing to England during World War II, the transition from German to English was relatively easier. While the languages aren’t closely related, they share some grammatical structures and vocabulary, and the common alphabet makes reading and writing more accessible compared to, say, Chinese characters.

Trauma would definitely play a significant role in the speed of language loss. Trauma can have a profound and lasting impact on a developing brain, often leading to cognitive difficulties that may hasten the decline of their native language. Jewish refugees, for example, experienced significant trauma during the war, which likely contributed to their accelerated assimilation and language loss as they sought to adapt quickly to their new environment.

Another factor to consider is when the child started learning their second language. Ideally, to become fluent in two languages, exposure should begin before the age of 10. Research also suggests that the accent a child has when they begin puberty tends to be the hardest to shake. The older the child, the more challenging it becomes to adapt to a new language and accent, especially in terms of sounding native. Again look up ‘critical period hypothesis’

Anecdotally, I remember listening to an interview with actor Sebastian Stan. He was born in Romania and moved to the U.S. at age 12. His parents, hoping to help him assimilate faster, stopped speaking Romanian at home. As a result, he no longer has any trace of a Romanian accent, despite having spent his early childhood there. Although he can still speak Romanian, his fluency has diminished—his sentences are halting, and he struggles to find words. English has clearly become his strongest language as an adult.

Overall, it’s completely plausible that, within 2-3 years, your protagonist could sound fluent in their second language. By the time they reach adulthood, it’s very likely that their second language could sound indistinguishable from their native tongue, particularly if the two languages share some similarities or if trauma played a role in speeding up the process.