Check a .po file:

$ msgfmt -c -v -o /dev/null file.po

Wipe out a .po file and keep only translated messages:

$ msgattrib --translated -o cleaned.po messy.po

Delete duplicate messages in a .po file:

$ msguniq -o no_duplicate.po duplicate_content.po

Show differences between two .po files (thanks to Jérome!):

$ msgcat -u file1.po file2.po > diff.po

Generate a .mo file from a .po file:

$ msgfmt -o target.mo source.po

If you want to have command-line tools use raw string instead of localized ones, add the following directives to your ~/.bash_profile:

$ export LANGUAGE=C
$ export LANG=C
$ export LC_MESSAGES=C