I think what parent meant was that Euler spent a few decades in Russia because of the funding provided by the empire. He spoke fluent Russian, even though there was a large German-speaking community there.
But it was typical for scientists to travel far for money. Some of the Bernoullis, a family famous for mathematicians, also worked in Russian for quite a while.
Does it really matter who payed 'em and what languages they spoke?
I wonder what he would have considered himself - he spent more time in St Petersburg than anywhere else but was born in Basel and spent a good bit of time in Berlin. For what it's worth, the Opera Omnia are published by the Swiss Academy of Sciences.
But it was typical for scientists to travel far for money. Some of the Bernoullis, a family famous for mathematicians, also worked in Russian for quite a while.
Does it really matter who payed 'em and what languages they spoke?