This is an English-language site. So, naturally, you expect English.
But it's also a "world" site. Majority of people here are from America and Europe, along with folks from the rest of the world (according to locations people revealed in their settings and data shown on a Perlmonks stat page).
For Europeans, it's pretty natural for people to communicate in multiple languages.
It's up to the author's discretion to write an "effective" post, including what language he wants to write it in. (Well, after all, the periodic table was invented by a Russian. We'd miss out a lot if we ignored something unless it's in English.)
's the aforementioned Perlmonks stat page. Could be slow at times.