Many technical works are carried out all in English- it is the lingua franca of programming. It is not particularly uncommon at all for foreign teams to work, code, and document exclusively in English. For example, look at the Lua project, written by a bunch of Brazilians. Documentation? English only. Filenames? All English. And the PLC code would not support Unicode either, so it's not like the Iranians had the option to work all in Arabic.
It does surprise me that it didn't hook CreateFileW, but I suppose that it knew in advance that WinCC only uses CreateFileA.