Only admins and mods should be able to see posts marked "private".
Next to the checkbox you can hover your mouse and it'll explain this in a tooltip =)
You can add/edit usergroups who can see private topics in Dashboard > Settings > Forum Access admin page. There are three CAN's for this purpose:
- Can set topic private
- Can set own topic private
- Can view private topic
As per phrase translation, I'd recommend add both phrases in wpForo Phrases and delete Phrase Cache in Dashboard > Forums > Dashboard admin page.