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Mathematicians of the 18th century and the Swiss people are known to speak and write every language.
Leonhard Euler belongs to both of these categories and wrote articles in any language, I am not able to track down any information on his native language. Is there any information on that? Or did he already speak French, German and Latin at the day of his birth?
I suppose that Euler's first language was German. Probably the Swiss German dialect, which they speak in Basel.
I say this because: a) he was born and went to school and to the university in the German-speaking part of Switzerland (Basel), b) His mother's (Brucker) and brother's (Johann Heinrich) names sound definitely German, and c) he had a German wife (Katharina Gsell whom he met in St. Petersburg), d) He was a Protestant and his father was a Protestant pastor, e) Euler is a German name. There is little doubt that he also learned French in his childhood, (his thesis is written in French) as well as Latin, Greek and Hebrew at school. Later he learned Russian and English. There are indications that he preferred German in daily life.
Following the custom of the time, he wrote his mathematical papers in Latin, papers on other subjects in German and French.
Here is a reference that confirms my guess:
Leonhard Euler and the Bernoullis: Mathematicians from Basel By M. B. W. Tent, p. 210.
