Why should I play this instead of Kancolle?
Even if you would be too much bothered with downloading a tool that translates Kancolle, it makes no difference. You can't understand a word in Milihime nor Kancolle.
So what would be a good reason/motivation to play rather Milihime than Kancolle?

Not that I mind you posting the game. I'm just missing some details/reasoning in your post e.g. to explain "more polished combat engine".