It's a lovely name. I pronounce it basically like mee-LE-na, but with my accent, it sounds more like a blend of mee-LE-na and mil-EN-a.
No, I don't think it would seem trendy on someone who isn't of a Slavic or Italian background. It isn't trendy where I live, but it's well-known enough that it wouldn't seem odd to meet someone named
Milena whose ethnic background was neither Slavic or Italian.