> I wonder if anyone knows a research on language complexity evaluation
> regarding textual processing?
In the last CSLP there was an interesting presentation by Philip Blache, "Evaluating Language Complexity in Context: New Parameters for a Constraint-Based Model". It doesn't provide a ranking, but explains the rationale for a reasonably sophisticated metric of complexity that can hold across different contexts. You can find it in the online proceedings at http://control.ruc.dk/CSLP2011/temp/CSLP2011Proceedings.pdf
> Intuitively, I can, for example, assume
> that English is easier for text processing than French because the
> latter is more inflected than English which would require more complex
It all depends on what kind of processing you're interested in. Anything involving the semantics, let alone the pragmatics, of natural language in context is so far beyond our ability to process it that measuring its complexity seems a pointless exercise.