PISA 2000 and 2003
PISA 2006 and 2009 (more detail here  and here 2009])
|Pisa sample question|
Data from an international test on how parents' education levels affect children's literacy scores, from here. I think we can easily extrapolate this to all nations.
There are videos from the Children of the Code that are showcased in the "breaking the spell" page that give even more data. Empirically and intuitively speaking, I think I have seen too many students (some of them being VERY intelligent, as tested by school psychologists, that show reading (and virtually always) writing disabilities (the inability to spell correctly more words than many).
Reports about budgets in different countries (most of them don't mention anything about the language issue though, but that goes without saying)!