1 Why do you need a separate menu entry to put it in the forground? Just press cmd+tab. Seems a lot simpler to me.
2 Dont know what that means. You can definitely add words that are not already there -- that's the whole point of the add owrds feature
3 ??
4 calibre comes with no builtin french dictionary, you have to install your own IIRC the only builtin dictionaries are for english and spanish, the two top languages of calibre users according to the calibre usage stats.
5 Are you saying the current translation is incorrect? If so you are welcome to contribute improved translations, see
https://calibre-ebook.com/get-involved