Copy the english language folder and paste. Rename the folder with your language name.
Add your translations in custom_lang.php. You can copy everything from the default_lang.php initially.
Finally set this language in application from Settings>App Settings>Localization>Language.
Never change anything in default_lang.php. Always change in the custom_lang.php file to prevent overwriting after updates. New updates will overwrite the default_lang.php file.
How to change any text?
Please go to \app\Language\[your language] directory.
Copy the default_lang.php and paste all to custom_lang.php