[Reproducible-builds] Weird localedef failures

[Esa Peuha]

Many apertium language pair packages fail with

localedef -f UTF-8 -i en_US ./debian/tmp/locale/en_US.UTF-8/
character map file `UTF-8' not found: No such file or directory
cannot read character map directory `/usr/share/i18n/charmaps': No
such file or directory

What is causing this? The directory /usr/share/i18n/charmaps is part
of package locales which should be pre-installed in pbuilder chroot.