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I just found out that my request may not be related to language string, which by the way, I'd suggest you guide users to do via language override.
What I have just noticed;
When a user joins a public group the stream looks good and makes sense, IE: Jules joined the group Open Group
BUT
When a user joins a private group the stream gets published but doesn't make sense at it doesn't display the group name, IE: lisa joined the group where it should display lisa joined the group Private Group
So the question is; which file should be modified in order to have the stream displaying a private group name just as it does with public groups?
We have been looking into this bug on our development servers to find a fix but are unable to replicate the error now. I am un unable to access your site to check it locally so could you please update your site details or let me know if this issue still exists.