Petra may send messages regarding tribute amounts. While in English it is not necessary to use plural in those strings, in some languages it is.
The issue has been reported at Transifex by GunChleoc: https://www.transifex.com/wildfire-games/0ad/translate/#/$/105312248?issue=yes
It affects some other strings which have also been reported.
And there is a trac ticket: https://trac.wildfiregames.com/ticket/4597