Amerikanski is used in those languages to indicate ownership (eg. This is an American car) so to someone of slavic speaking origin this would be wrong (they would've said Amerikanci to indicate plural). But even aside from that, this honestly sounds so put on. Even the worst english speakers I know from eastern Europe know that you add an "s" to form plural. We all take English from elementary school and nobody would make that mistake, not to mention that it's a mistake even in a slavic language.

A much more common mistake you will find is not knowing when to use "the" or "a".

Edit: unless it is a Russian pretending to be an American who is pretending to be Russian which, who the hell knows, anything is possible.