Flags as icons?

Sorry, flags are not icons in the FontAwesome font library. I can see it was requested by someone here, and the FontAwesome folks said it’s not something they intend to add.

Sticky Side Buttons plugin uses the FontAwesome icons. Adding custom icons is not possible within the current scope of the plugin. It would be possible for a developer to modify the icon library used by the plugin (to use a flag icon library for example), but it’s significant development work – at that point, you’d be much quicker to just code the buttons into your theme as an html/css element. Also, it might be worth checking what solution your multilingual plugin/solution offers for this? Generally they have their own language picker as part of how it works.