For example:
http://i.imgur.com/JKS7SmN.png

Possible solution:
Check if number of unread mails == 1, if so display "message";
else display "messages";

I think it's fine as it is.

Still a bug, even if low priority one

I wouldn't even interpret it as a bug. Doing this is very common. It says "1", and not "one", or "a". In that context, number + pluralized form is completely fine from my understanding, even if it's just one message.

I mean.. If this is the kind of thing people say is wrong with the project, I think we did a decent job.

It's not that easy to fix (as suggested) because the messages are configurable. In my own private plugins, I add a configurable placeholder '%plural%' after 'message', i.e. 'message%plural%' and if messages != 1, %plural% is replaced with 's' otherwise it's replaced by an empty String.

I do agree that it would be nice if this was 'fixed'.

Sounds pretty ethnocentric ;)

Uhm sure but Essentials was created by English developers... The fact it has support for multiple languages itself is good enough. Whatever, if you don't want to do this then just say lol.

Such a %plural% placeholder is not compatible with every language.

Then only make %plural% in the languages that have the rule '1 is singular'?

Plurals are hard