I have a "pt-BR" locale. When I call it, it works as expected, downloading the /pt-BR/locale.json file as expected. But when I open the console there are two 404 requests calling for "dev" and "pt".

Is it a normal behavior? By calling only "pt-BR" should't it just request that specific file?
default behaviour of i18next is to resolve translations in order:
1) de-DE (lng plus regional diffs)
2) de (lng)
3) en (defined fallbackLng)
so getting that not found is 'normal'.
you can change that behaviour with options:
@jamuhl worked as expected. Thanks :+1:
Most helpful comment
default behaviour of i18next is to resolve translations in order:
1) de-DE (lng plus regional diffs)
2) de (lng)
3) en (defined fallbackLng)
so getting that not found is 'normal'.
you can change that behaviour with options: