Set the back-office theme to a specific url (On drupal 6)

On some of the websites i have to manage, the translation page of a node ( mywebsiteurl.com/node/NodeNid/translate) use the main theme instead of the back office theme.

I have some ideas about how force the back office theme, but i doubt it's the right way.

I create a specific module and use a preprocess_page function to force a 

$GLOBALS['custom_theme'] = variable_get('theme', 'mybackofficetheme')  


if i am in a translation page. But it doesn't works, so can you suggest me the best practice?
I'm not familiar with using translation pages, but I've had good success using the themekey module for changing themes based on URL:
https://www.drupal.org/project/themekey