I find it irritating that I can't delete messages sent via my BOINC manager
that I've read or can't read because not in a language I understand.

Many of the messages once read are no longer needed, so our being able
to delete them would also take care of the mulitple language messages.
They just clutter up my list.

Also, some of the messages we receive are like preaching conversion
to the already converted.

Why not start a campaign requesting all participants post to their
social media pages about BOINC and volunteer computing with a link
back to you for you to explain the what, how, why and sign-up here.
You could even provide a short, little blurb for us to use with the link.
That would make it fast and easy for people to just copy and post.
It would get the word out and presummably a percentage, even if
small, would sign-up. A small percent of enough posts would add up.