While poking around for some references today I took a detour through the Mathematical Reviews. I heard that Rota had written some zingers, but one of the first of his that I saw, on the Riemann-Liouville integral in the complex field contained the following comment:
Many of the results are essentially known, but this seems the best treatment in the literature, which would well justify a previous brief study of Esperanto for the interested reader.
A quick investigation shows that the paper is indeed in Esperanto!
The journal is Funkcialaj Ekvacioj, which presumably means “functional equations” in Esperanto. The entire editorial board is Japanese, and since its inception most of the contributing authors have been Japanese. The journal has been going since 1958, and they have full archives online (vol. 1-40 are free). At the beginning, they had articles in English, French, and Esperanto, with the abstract (if necessary) translated into Esperanto. That stopped as early as the 1970s (more’s the pity), and articles are just written in English, but the journal’s title remains in Esperanto.
This really made my day — maybe I should translate an article into Esperanto… except that I’d have to learn Esperanto first.