Four key issues revisited and correlated

1> Identifying colloquialisms
2> Identifying the contexts
3> Translating to non-colloquial forms w.r.t. contexts
4> Extracting (possibly) new information from the non-colloquial forms

Extension:- Conversion from non-colloquial to colloquial.

Tasks 1 and 2 are related in the sense that sometimes some expression may turn out to be colloquial or not, depending on the context. Tasks 2 and 3 are related, because you can't do the translation unless and until you know the context! Similarly, tasks 3 and 4 are also related, for "new information" definitely implies something that was not there in the context, or that "enhanced" the context in some way. So it does depend on the previous translation step. Any discrepancy in the previous step will baffle this step.