[DL] decidability

[Ward]

Dear all,

I am trying to understand more of Description Logics and I have a 
question about decidability. If I understand it right then decidability 
was originally only used for problems or queries or something like the 
satisfiability of a class. Now a logic language is called 'undecidable' 
if it is possible to construct an ontology with it, about which you can 
pose undecidable questions.
What does it mean for a logic language to be undecidable? Is it a bad 
language? Is it not the task of a reasoner to point out whether a query 
is decidable, instead of restricting the whole language?

Thank you very much,

Ward