The default locale for our themes is en_US. If you want to change a translatable string in the theme without changing the theme locale, you must make a copy of the languages/default.po file, and rename it following the locale convention (e.g. en_US.po for english, etc.).
If you want to localize the theme, you should edit the wp-config.php file and change the WPLANG constant value to something different (e.g. de_DE for german), and create a new .po file in the aforementioned folder as explained above.
If you’re using a translation plugin instead, such as WPML, make sure to always create .po files according to the languages that have been enabled.
