The Debian package gettext includes a whole set of command-line utilities for handling i18n material.
Several utilities may help translators in manipulating the PO files contents. For all these utilities, see their respective man pages for details about their use:
msgcat combines together one or several files. When used on a single file, it will wrap the file contents and make it easy to read.
msgmerge may merge together translations from
one or two files as well as update a PO files according to the
contents of a templates file.
msgfmt is aimed at compiling message catalog files but is also very useful for checking files and get statistics about their contents. The use of msgfmt --check --statistics --output-file=/dev/null <language>.po is highly recommended before commiting a translation file or sending it to the package maintainer.