Because of I can not comment Manuel's post I answering here in forum.
Support for plurals at browser side was added in #Hubzilla
function uses there too.
with (correct it if I made a mistake in logic):
"Plural-Forms: nplurals=2; plural=($n != 1 ? 0 : 1);\n"
one more time.
It will fix this (just checked).
!Hubzilla Support Forum