/usr/src/glibc/debian/debhelper.in/libc-l10n.install is in glibc-source 2.23-0ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | debian/tmp-libc/usr/share/locale/[a-z][a-z] usr/share/locale
debian/tmp-libc/usr/share/locale/[a-z][a-z]_[A-Z][A-Z] usr/share/locale
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