There is something wrong with Plural-Forms
in fra cake.po file.
Change:
"Plural-Forms: nplurals=2; plural=(n==1? 0 : 1);\n"
to:
"Plural-Forms: nplurals=2; plural=(n > 1);\n"
and it should work.
I've tested it on:
echo sprintf(__dn('cake', '%d year', '%d years',2),2);