Well yes I think in English now, but that wasn’t always the case, and I have reverted to thinking in French whilst living in France.
But I wouldn’t say that a ‘consultant geriatrician’ is necessarily an expert on the English language.
“A first language (also native language, mother tongue, arterial language, or L1) is the language(s) a person has learned from birth[1] or within the critical period, or that a person speaks the best and so is often the basis for sociolinguistic identity.”
French is my native language and mother tongue, and therefore my first language. Spanish is my paternal tongue and my second learnt and second cultural language, therefore my second language. English is my third language, that fact it’s the only one I now speak I don’t reckon comes into it.