I could be wrong. This is a complete guess, but my suspicion is that Yandex Browser is doing some very lazy checking of a user's language preferences and defaults to Russian if the visited site is not available in the user's preferred language[s]and user hasn't got 'en-US' defined as a fallback.
[My language preference are set to: English UK > Irish > Spanish > English]
en-GB,en;q=0.9,ga;q=0.8,es;q=0.7
I could be wrong. This is a complete guess, but my suspicion is that Yandex Browser is doing some very lazy checking of a user's language preferences and defaults to Russian if the visited site is not available in the user's preferred language[s]and user hasn't got 'en-US' defined as a fallback.
[My language preference are set to: English UK > Irish > Spanish > English]