I personally thought that Uk/Us (and western europe?) just borrowed the name and applied it to girls. I wasnt aware sasha was originally also the nickname for alexandra. But it makes sense obviously.
I guess "our" (English/other germanic languages) made Sander and Sandra from Alexander and Alexandra. And ofcourse Alex and Xander, And I guess Lex (and Alexia but that seems so forcefully made up imo)