There are a number of tools you can use to translate WordPress projects. However locale packages are built from the translations on translate.wordpress.org (that runs on GlotPress). So no matter which tool you choose, after you finish translating, you will need to go to translate.wordpress.org and import the relevant gettext files.
You can get to know the tools you can use in the descriptions below:
GlotPress will let you, or an entire team translate your favorite software. It is web-based, open-sourced, and completely in sync with the main repositories and the preferred method for translating WordPress into your language.
The official Gettext tools package contains command-line tools for creating POTs, manipulating POs, and generating MOs. This tool is for those comfortable with a command shell.
An open source program for Windows, Mac OS X and UNIX/Linux which provides an easy-to-use GUI for editing PO files and generate MO files.
The Ubuntu Linux project has a web site that allows you to both translate messages without even looking at a PO or POT file, and export directly to a MO. In general, the WordPress projects on LaunchPad are out of date. If you’re working on a current translation of WordPress, be sure to contact the team before making changes to the translation in LaunchPad.
An open source web-based translation system. The server hosted at locamotion.org currently has WordPress translation enabled on it. As with LaunchPad, check with the current translation team before using Pootle.
Another open source PO editing program for the KDE window manager on Linux.