Hello is a very nice and fast application thanks.
Which file I need to change to translate the buttons to another language

You can use browser's native buttons so you can change the appearance with css.
To do so, open web/codelib/sys/CHtmlMacro.inc.php in a text editor. Find the following line:

define('TYPE_NORMAL_BUTTON', 'rollover' );

Change rollover to html. The result should look like:

define('TYPE_NORMAL_BUTTON', 'html' );