[Birdman]

Thanks mate. I tried to do exactly what you told me. I didn't have a hud_msg.properties file under files/i18n, nor did I in my Mods folder... so I used AAA SFS Extractor to extract the hud_msg.properties and hud_msg_ru.properties from files.sfs. I only found the second one (hud_msg_ru.properties), so I made sure it was written in English language, NOT in Russian (opened it with Notepad), renamed it hud_msg.properties (WITHOUT "_ru") and put it in my files/i18n folder. Launched the game but... the trick didn't work. =(
So, if neither hud_msg.properties nor hud_msg_ru.properties are meant to control the hud language in-game. what kind of file is? What should I change in order to get English language back?
I'm very confused now and don't know what to do... =(

That file is not in Files or MODS folder, look at the i18n folder directly in the root folder of the game. Copy both hud_msg.properties and netmessages.properties to Files/i18n and add the _ru to both names, and you'll get english texts both offline and online.