When I asked a question on algebraic geometry, some people referred to EGA as its answer. I think it's perfectly OK as long as there are other answers which do not require you to be able to read French. But what if it's the only answer? My French is not very good, so I prefer English papers or books. Moreover I wish most of students understand the answer. Hence my title question. I'm a Japanese, so I have no idea how much French is easy for English speaking natives.
Reading French for a native English speaker is not particularly easy if they haven't had any experience in it, but it is much easier than for someone whose native language is Japanese.
However, in algebraic geometry, EGA is a very canonical reference. Furthermore, there are many other influential French-speaking algebraic geometers, such as Serre and Deligne. Finally, many French-speaking mathematicians continue to write in French (whereas most natives of other countries write in English nowadays, whatever their native language). For all these reasons, most students of algebraic geometry learn to read enough French to muddle through math papers written in French.
There are some alternatives to EGA/SGA that are written in English, but not everyone knows them, and so you can't really expect people to cite them. EGA remains the fundamental reference for many people.
I don't know that you have to assume they know a language to list a very important text in the field that is in that language. More information is better. If someone is sufficiently motivated they will learn the language. I wouldn't want to assume someone is not motivated to that extent. Additionally, in the case of English speakers and French, enough English speakers learn French in school that there is a nontrivial probability that they know French.