[Plugin: Front-end Editor] How do I change language?

Go to wp-config.php and change this line:

define('WP_LANG', 'en_EN');

to something like this:

define('WP_LANG', 'de_DE'); (german)

If the language you want is included with the plugin, it will work instantly.

If not, you will have to make your own translation files, as described in the links you mentioned.