For localization, phpMyAdmin uses Gettext; you can find po files for each
translation in the po directory in phpMyAdmin sources. You can
translate them using the usual tools for handling Gettext translations or use our
translation server.
The phpMyAdmin's documentation is being translated using Sphinx and gettext (see documentation for existing translations) and the process here is pretty much the same as translating phpMyAdmin itself.
It would be a good idea to subscribe to the phpmyadmin-translators mailing list, because this is where we ask for translations of new messages.
The translation server includes various subprojects, you should always focus on
the latest stable branch. Once this one is complete you might want to continue
translating master (which stands for the current development branch)
or documentation. The changes are automatically propagated, so you don't have
to fear having to translate a message more than once.
As phpMyAdmin uses standard Gettext files, you can use your favorite editor to edit them. There are various editors for translations which you can use for offline translating.
You can download po files either directly from Git (see developer instructions for more details) or use the translation server, which allows you to directly download the latest po file of the desired version.
To submit your changes, you can either use the translation tracker on SourceForge.net or the translation server, which can merge back your changed po file to the existing sources.
You can find more information on our wiki.