OK, for those, who like to change locale at piCore, getlocale.tcz from main repo works fine.
It need the following extensions to be loaded manually before generation: dialog.tcz, glibc_apps.tcz, glibc_gconv.tcz, glibc_i18n_locale.tcz and for boot into the new locale it than need only glibc_gconv.tcz which the generated mylocale.tcz already has in dep.