Menu language doesn´t change

Hello,

even after updating to WP 4.51 everything worked well.
But now the menu doesn´t switch in the frontend.

I can switch the language of the page into another one. But the language in the menu doesn´t change.

I am using the User Language Switch on another Website, where it is working fine. I compared both installations, there are no differences.

Could it happened, because i changed the website from http:// to https/?
But even after updating the database with an older one, the language of the menu doesn´t change.

What can i do?

Thank you,
Michael

https://wordpress.org/plugins/user-language-switch/