What do you mean by "it doesn't work" ? You can't save the override file ?
If that's the case, then yes, it's probably a permission issue.
But if you can, then the problem is elsewhere. We didn't change anything on that end in the latest version and it's still working for everyone else. So I can only think that you did something else on your end which led to that.
Joomla 2.5 has an editor for the translation override file, so you might want to try it:
demo.hikashop.com/administrator/index.ph...uages&view=overrides
It is compatible with the HikaShop override file.
Also, you will find in the configuration of joomla the "debug language" option which will give information on errors in translation files if activated. You will need to look at the bottom of the page for the language traces.