If I understood well, what you want is to use the Custom Block Manager plugin to create blocks of content, but the content of the custom blocks is not multilingual (which I believe it should!).
There's already a a BUG filed in Bugzilla, so they can think of a solution for this.
This would be a very nice feature indeed.
If you're in a hurry, here's a workaround
I'd suggest creating your own Block plugin, instead of using the Custom Block manager, as this will allow translation of its content, if it's not that complicated.
Check the code of those types of plugins and see if one of them helps you create what you need.
Ramón Martins Sodoma da Fonseca
Analista em C&T
Coordenação de Tecnologias de Informação
Instituto Brasileiro de Informação em Ciência e Tecnologia - IBICT
Ministério da Ciência e Tecnologia - MCT