Do you have own picture for each languages?
Example.
Customer open login page and saw Facebook German picture, he changes language and saw English picture.
You can do it without me.
Each picture (logo) was described in external CSS style. You can add your own CSS style and each logo will have own picture.
Example.
You should add code in PublicInfo.cshtml.
@{
var workContext = Nop.Core.Infrastructure.EngineContext.Current.ContainerManager.Resolve<IWorkContext>();
if (workContext.WorkingLanguage.LanguageCulture=="Ru-ru")
{
<style>
.social-facebook-btn {
background-image: url("images/facebook-account-logo-ru.png");
}
</style>
}
}