In new api, the context in encapsulated and propagated with 'env'. There is no need to add in the signature. Method decorated with api.multi/api.model replace method(self, cr, uid, context).
The module did only work in english because the themes were found
amongst the applications thanks to their categorie names.
Solution is to search categories thanks to their xml_id :
base.module_category_theme or base.module_category_theme_...
and not
base.module_category_hidden or base.module_category_theme_hidden
Yannick Tivisse