So, i’ve found a way to resolve this, so i figured out that i would write it here for the next person.
In order for Lingotek plugin to copy original post featured image in the translated post, and also not remove it from the original post, you must go into Lingotek translation menu, “Manage”, then “Custom Fields”, and change “_thumbnail_id” to “Copy”.
So, okay, i feel kind of dumb right now… At least i hope this topic could help others…