Add Flags

Yes it is possible but need a little knowledge in css to adapt the look to the look of your site and content strategy. This (old) post will give you some ideas.
https://2011.wpmu.xilione.com/en_us/130/themes-header-example-step-by-step-of-languages-nav-menu/

In your case, the menu exists, just look at css part and adapt selector (id class) to your context.
Note: do not forget that flag is a country and not a language ??

Good work

ok, but does the theme include de flags png? or shall i upload my own flags?

via ftp inside a subfolder of current theme like images/flags…
in css, the subfolder must be the same !

M.