This article discusses the myth of bilingual people reverting to their native language when expressing strong emotions. From what I’ve read, it’s not necessarily untrue that this happens, but there’s more nuance than automatically reverting to one’s earliest language, and more variables than time.
A couple quotes I found interesting:
When a childhood in one language lacked affection or was marked by distressing events, then bilinguals may prefer to express emotion in their second language.
When bilinguals are angry, excited, tired or stressed, their accent in a language can reappear or increase in strength. In addition, they often revert to the language(s) in which they express their emotions, be it their first or their second language, or both.
I read the article as part of my research for Stars Fall Out, and it’s not totally applicable in my case, since I’m writing a multi-lingual character who starts slipping out of an assumed accent. But this is still useful information to have it mind.