Article associations for languages works right, maybe one thing could be nice to change.
If new article is not saved you can't see associations fields at all, after save you can see other languages association fiels appear. Wouldn't it be nicer, that associations fields become visible at creation and then after save disappears only field related to article language.
But the main problem is "Category associations" field
Added it to category form, but it shows always only articles associations, not category.
After new category save things doesn't change.
Tried to remove and add to form this field several times, nothing changed.
As you can see here, right now it is hardcoded to associate just things of the "content" table... Also if you look at the code of the cck_storage_location joomla_category plugin, you will see around the line 323 this:
I think this is something that will be developed in the near future but for now is not supported unless you develop your own cck_storage_location and your cck_field plugins...