So how exactly do you determine the content is in Russian?
Entonces, ¿cómo se determina exactamente que el contenido está en ruso?
ak więc dokładnie ustalić, że treść jest w języku rosyjskim?
Так как же определить, что контент на русском языке?
那麼如何確定內容是俄文的呢?
As you can see we have text in five languages in this comment. Because of the way UNICODE text works, there is no specific need to declare text to be in a language, unlike in the days we used ANSI character sets and code pages.
While you might like to do this, how to determine what is Russian is the serious question. A simple filter using a couple of Cyrillic characters would be fairly effective right now, without waiting for someone to invent a method. It would not be specifically Russian, but Cyrillic characters are generally not used in English so if your desire is to remove languages other than English, the fact other languages that use the same alphabet like Kurdish will be caught up is not going to be a problem.