different centres responsible for the native language and the language learned later.
That's very old news. Brain scans have shown that the brain processes a person's first language(s) in one way and languages learned later in a different way.
most people still count in their first language
It's more that people count in the language they learn basic arithmetic in. Several months ago in my local Biedronka a couple of children (8-0-ish) were speaking in russian to each other but higher numbers (like osiemdziesiąt) were in Polish.
Similarly when Maria Callas (bilingual in English and Greek and with near-native Italian) was asked about language she said "I count in English" (the language she learned basic arithmetic in during her NYC childhood).
It's also why speakers of many minority languages (especially indigenous ones) or speakers in post-colonial areas often use higher numbers in a language used in education (IINM Greenlanders mostly count in Danish and not Greenlandic).