I'm pretty sure there was a thread about exactly the same thing a couple of months ago if you search. I personally think it's ok and am not keen on Valentina either but the only Valentine I know is a man.
It's a boy's name isn't it? I doubt you'd be able to get anyone outside of your immediate family to pronounce it the way you want either. Everyone will just pronounce it the way it's normally said in English. My dh is French so we picked named that are pronounced pretty similarly in both languages as it is a pain constantly having to correct people.
As you say' it's a marmite name but worth bearing in mind when everyone mispronounces her name and thinks she's a boy.
Girl's name in France, pronounced as you want; but a boy's name in England, pronounced as in Feb 14th.
I wouldn't be that shocked to meet a girl called Valentine in England, but I would pronounce it the English way, not the French.
DS1&2 both have French names - when we're in England people pronounce them the English way and we suck it up because we think it's normal that different language = different pronunciation. But if it'll bother you then think again, because the vast majority of people will pronounce -tine not -teen.