Any Sellacious form can be extended by a plugin, and anyone who is familiar with Joomla plugin development can do it for you.
And Sellacious Backend Categories can be Multilingual through a paid plugin which we have already developed and will release it very soon.
Yes with the plugin you can have multilingual Sellacious Backend Categories as of now but for attributes, you may have to wait for little more. As we haven't created any plugin for that. And regarding the price we haven't finalize any amount yet, we will publish it pretty soon.