Great movie overall.
The relationship between Portman and Reanu isn't sexual in nature. Portman is a young girl that doesn't understand what her feelings are as she's going through that phase of life where hormones and physiological changes really do a job on the psyche. She sees Reanu as a lover, saviour, father, and friend. This is a dominant adult male that shows honest concern for her well-being, without being verbally or mentally abusive, and as this is a first for her, she equates her appreciation for all that he's done as the feeling of love. She doesn't know how to handle the sudden and violent deaths of her family, or the anger of the bad cops, but she does know that Reanu makes her feel safe and protected and appreciated.......and maybe that's all that love really is. For the first time in her life, she has a man that talks kindly to her and spends "quality time" with her, and includes her in his life and hobbies. That type of inclusive behavior, as demonstrated by Jean Reanu and, I would argue, good fathers around the world, could, and does, form very strong feelings in a newly pubescent girl.
As to the rest of the movie, I enjoyed it. There is a lot of stuff that might not be realistic, but that's half the fun. And I definitely loved the ending with the grenades. That was a very noble way to go!
PS: the guy in the park was hit with a red paintball and survived. Who the guy was supposed to portray is anyone's guess.