Hi @chrystl!
I tried reinstall the plugin and I assigned all of the posts/portfolios/pages to the default language (en_GB) but still, none of the portfolios is shown. Also now, there is no drop down menu in the header, where I could select the language.
Thanks for the help. The solution for my problem was quite strange. I could easily installed and activated the plugin called Mailchimp for WordPress Lite, but when I tried to install it from the recommendation ones, I could not do it. Anyway, thanks again for the help!
