Pop-up title not multilingual

We already contacted the support-team of the concerned plugin (Timetable WordPress Plugin – Weekly Class Schedule from Curly Themes) and according to them You may help us. Their plugin adds a field called “class” in “wordpress post type” in the settings. (screenshot) In the backend you have the same fields and options to design a class as a normal post (screenshot). Now when you click on a class on this site it opens a pop-up. In this pop-up the content is multilingual and changes to the requested language, only the title does not change properly. (screenshot) We already deactivated evry other plugin we use with no success. Could You please help us? We do not know where to look else for the problem.

Thanks in advance.