How to translate or edit "An error occurred" page?

[motomac]

Majority of my users don't speak English, so they don't understand this message. What is the best way to translate it into another language?

[Ilya Hoilik]

1. Make copy of error.html called error-custom.html and also of upgrading.html called upgrading-custom.html
2. Translate these pages into your language
3. Open constants.php and add these lines

define( 'ERROR_PAGE', 'error-custom.html' );
define( 'UPGRADING_PAGE', 'upgrading-custom.html' );

This should resolve your problem