i18n improvements in Message notifications

Add singular form of “you have x new private messages”, and use
`bp_core_number_format` to format the number correctly for the locale.

See #5767, props SGr33n

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8655 cdf35c40-ae34-48e0-9cc9-0c9da1808c22