Writing good subtitles is a bit like translating a novel or other fiction, it is more important to convey the meaning or sense of what is going on than translating word for word what the original language was. When translating idioms and jokes in particular do not translate well 'word for word' but there is often an associated phrase which has a similar meaning - a well translated book will work hard at getting a similar feel as well as a similar meaning to the original
Subtitles are slightly harder than the written word, as it looks slightly odd if the length of text and spoken word do not correspond to some degree
In the example you are giving here, it was probably only important to the translator that a jokey pun of some sort was included, to convey the sense of what is going on. It is possible that the French joke does not even refer to 'Russian' - which as you say would only work in English.