German special characters in Slide In translation not showing correctly

Hi there,

For a client I need to translate the Slide In plugin. Everything is working well. On the front end the plugin shows up completely in German. Except of one word. The German word for "Close" is "Schließen". However it is shown on the front end as "Schliessen". Which is a wrong spelling and not acceptable. I have tried already to use the html code "ß" instead of writing directly "ß" in the translation file. But that resulted also in "Schliessen" on the front end.

I would appreciate it if somebody could look into this issue and help me finding a solution.

Thank you.