I don't particularly like
Luka on either gender, but can understand how it could sound like a feminized version of
Luke and could have some appeal that way, but at the same time, I think it should be left for the boys... Though I'd prefer
Luke or even
Lucas (I don't really like
Lucas, but prefer it to
Luka which feels like something's missing). There are so many nice ligit girls' names that there's no need to snag another boys' name and pin it on a girl... though, if one must, perhaps
Luca could be used as a nn for
Lucinda and then when she grows older, she could decide if she wants to continue w/ the nn or go by
Lucinda or
Lucy.