First of all you should create new topic for every separated problem. It simplifies information search for other forum's users.
Next, frankly, I don't know why did you translate Mibew into French. There is French language package here:
https://mibew.org/download. Moreover it contains a few localized buttons which you can use.
Now about the custom buttons. You need to create images with names like
<style name>_on.png and
<style name>_off.png and put them into locales/
<locale code>/button/ directory. You can find templates for buttons at:
https://github.com/Mibew/design/tree/master/design. That's all. Just don't forget to replace
<style name> and
<locale code> with real values.