Just login to our site with the same username that you have purchased the theme and you will be redirected to your account page where you can find the download section to download the theme file that you have purchased. After downloading the file just activate it in your site as you did for the free version.
I have a second question: I have understood, that the pro-theme included a gallary of icons I could use. Is this correct ? And if yes, where do I find them ?
As our theme is by default an english. Did you change it on other language? Once you confirm from dashboard -> settings -> General -> Site Language -> Choose English (United States)
Do you mean multilingual? As our theme does not support multilingual. It needs code customization or you need to hire a developer to make your site multilingual. Recently we are busy on some other projects so multilingual features may be available on future time only.