In 3.10.1, the localization claws-mail.mo files were installed to $datarootdir/locale. This defaulted to $prefix/share/locale. With 3.11.0, they are now being installed to $prefix/lib/locale. The real bug is that ./configure now ignores --localedir if we want to set a normal localedir. ./configure --help still lists it as an option, but it's ignored when set.
Fixed in release 3.11.1.