When using the web interface, the language will be set to what the administrator chooses it to be. And on the first access to the admin pages, this will be chosen from the settings of the browser of the administrator.
But on commandline this information is not available, and this setting will be used instead.
If you want to run phpList in a language other than english, you will want to set this before running initialisation of phpList.
For valid language ISO codes, check the translation website.