The translations (po files) are utf-8 plain text files, which you can translate with any editor. However as the format is quite standard, there is wide range of editors, which will help you editing it. For example there is Poedit (standalone editor) or gted (Eclipse plugin). For sure you can find much more of them.
You can also use our translation server and do the changes online.
By default, every logged in user can translate, if you want to have more control over the translation, the privileges can be adjusted so that only selected users can translate and other ones only contribute. If you want to do so, please write to Nijel.
The changes made through web interface are comitted under your name to a local branch, which is then occasionally (usually every day) merged and pushed to master.
To add new translation for phpMyAdmin, you need to invoke:
msginit -i po/phpmyadmin.pot -l LANG_CODE --no-translator -o po/LANG_CODE.po
For the documentaiton, it is enough to add language code into LANGUAGES list in Makefile.