Everything (or at least almost) regarding language with phpGraphy is controlled using external languages files located in
base/lang/. The default and reference language file is the english one (
lang_en.inc.php) must NOT be edited. There is others languages available, courtesy of volunteers translators but they might not be always uptodate, if this is the case, then the displayed text will fall-back to the default english content.
Creating an official language file and customizing a bit an existing one is a different task. If you want to contribute to phpGraphy by submitting a language file, please visit the TranslatorPage of the Development Website, you'll find instructions on how to proceed.
If you just want to customize a bit of a language file, then read the previous paragraph Creating our own personal language file