Sorry my error, I did not tested appropriately all cases, but now the problem happens with all subfolders . So briefly if I am getting a folder (only with translations) , which is children of another folder I get that error.
@Harro Verton You're right, Model_Translation has not the belongs_to relation.
I resolved changing ORM to 1.8/develop ! Thanks
I share, what I discovered about the behavior of this error.
When I was fetching the folder, in that case for editing it on the form, I was also selecting all folders to put inside a select menu, to choose one of them as parent of the folder I am editing on the form.
I tried to delete temporary the select of all the folders with all related childrens, and I was not getting the error, and translations were correctly fetched .
The case was: if the Folder had not translations I could query all the folders for the select menu, instead if I didn't query two times the Folder model , I could get the related translations.
So there was definitively a problem handling maybe twice the the two relations.