The code of the default American English locale is en-US (en_US in some files). You need to change it to that indicating your language and country according to ISO standards. To find the required locale code, see the Appendix. Locale Codes section.
Changing the Locale Code in File Names
Change the locale code in names of the following directories, and the names of all files contained within:
You also need to change the locale code in the following entries found in the
/usr/local/psa/admin/plib/locales/en-US/messages_en-US.php file (on Linux) or the
%plesk_dir%\admin\plib\locales\en-US\messages_en-US.php (on Windows):