D&D continuity is a mess with its own contradicting lore and the Ravenloft setting suffers a lot for those discrepancies.
Not all sourcebooks agree on whether or not the different elven dialects are close enough to be understood by each other sub-races of elves. But generally, they do indicate all type of elves can understand each others. I believe it all comes from the early days of D&D, where a decision was made for the sake of simplicity that all sub-races of elves could understand each other. It is a silly rule, one which deprive us from the RP opportunities of elves from different cultures encountering each others.
Above all we play in the Ravenloft setting, we should be bound only by its rules (and even then we certainly have our fair share of rule variations of our owns within the mod). Sure it is nice to be accommodating as best as possible with the rules of other D&D settings, but we certainly don't have to be bound by them. We have not addressed the question of Elven languages from Non-D&D settings either. Some, like the languages of Middle-Earth are quite clearly different from each others, I fail to see why they should be able to understand all other elven dialects as well.
To avoid all issues we should put all languages on the same footing regardless of their setting of origin (D&D or otherwise), I propose the following rules:
1) Unless specifically noted otherwise, all racial dialects are considered unique enough to be their own different languages. Speaking one does not allow a player to understand the other dialects of his race. This applies even if this contradicts a non-Ravenloft sourcebook.
2) Darkonese elves speak the generic elven language of D&D [Elf].
3) Sithican Elvish & the Sylvanesti language from Dragonlance setting are one and the same [Sc]. But while the Sylvanesti speaks it graciously, the Sithicans are known to speak it in a dissonant way. They would still understand each other perfectly well.
4) The generic elven tongue [Elf] and Sithican [Sc] are still close enough, that speakers of those tongues may understand each other, but such communication requires a measure of efforts, concentration and to talk slowly to each other. //Here I would suggest a concentration check DC 15 in normal conversation conditions, DC 20 in times of stress (as in a combat).