Hi Riccardo,

Please try to add the phrase key with a backslash (\) like this:

You don\'t have permissions to see this page, please register or login for further information

Also please don't forget to delete phrase caches (Dashboard > Forums > Dashboard admin page) before checking. 

Hi Sofy,

Thanks for your last answer, but this does not work either (phrases cache deleted). Also, this phrase is not in the wpforo-*.po file.

So, how can we fix this please?

You've added wrong slash. The correct is "\" not "/". Please remove it. And create one with this key:

This topic doesn\'t exist or you don\'t have permissions to see that.

Also try using Loco Translate plugin

Sorry, I updated my post. The correct is this "\".