mo-mode.el is a spot of code for decoding and viewing Gettext .mo and .gmo compiled message catalogues, using msgunfmt. Get it from

The main use is for browsing installed message files, if you don’t have the .po source handy. Editing and rewriting such a file works (except out of TarMode), via the FileFormat mechanism, though editing would be unusual.

Filename completion (Emacs 21)

In a /po directory of a project you normally don’t want to visit the .mo files, only the .po ones. You can set .mo as an ignored extension so such files aren’t offered as completions (though you can still type it explicitly of course).

    (add-to-list 'completion-ignored-extensions ".mo")
    (add-to-list 'completion-ignored-extensions ".gmo")

Emacs 22 has this setup by default.

Gettext CategoryModes DataModes